Webmail worked fine yesterday. Today someone clicks the Roundcube icon in cPanel and gets a blank white page, or types their password correctly and just gets bounced back to the login screen with no error at all. If this sounds familiar, you're not dealing with a broken mailbox — the mail itself is almost certainly fine. Roundcube is a PHP application sitting in front of your mail server, and like any app, it has its own failure points that are separate from IMAP, SPF, or DNS.
Symptom: What Roundcube Failure Actually Looks Like
A handful of variations show up in support tickets, usually described the same vague way ("webmail is down"):
- A completely blank white page when opening
yourdomain.com/webmailoryourdomain.com/roundcube. - Login accepts the password, spins for a few seconds, then dumps the user right back at the login form.
- An on-screen error like "Failed to connect to IMAP server" or "IMAP Error: Login failed", even though the same credentials work in Outlook or Thunderbird.
- The page loads but looks broken — no styling, missing folder list, buttons that do nothing.
- A PHP warning or fatal error printed directly on the page instead of the login form.
The key diagnostic question is always the same: does the same email address and password work in a desktop or mobile mail client over IMAP? If yes, your mail server (Exim/Dovecot) is healthy and the problem is contained entirely inside Roundcube.
Cause: Roundcube Breaks in a Handful of Predictable Ways
Roundcube depends on more moving parts than people expect: a writable temp/log directory, its own SQLite or MySQL database for session and address book storage, PHP session handling, and a working connection back to localhost on the IMAP port. Any one of these can fail independently of the mail server itself.
| Symptom | Usual Cause |
|---|---|
| Blank white page | PHP fatal error suppressed by display_errors = Off, often after a PHP version bump |
| Login loop, no error | Roundcube's session/database file isn't writable, or browser cookies are blocked/stale |
| "Failed to connect to IMAP server" | Dovecot isn't listening on localhost:143, or a firewall rule is blocking loopback traffic |
| Missing styling / broken layout | Browser cache serving an old skin after a cPanel/Roundcube update |
| Fatal error text on screen | A Roundcube plugin incompatible with the current PHP version |
Fix: Work Through It in Order
1. Confirm the Mailbox Itself Is Fine
Before touching Roundcube, test the same address and password in a mail client over IMAP (port 993) or check WHM > Email > Test Authentication if you have root access. If the login fails there too, this isn't a webmail issue — go check the account's password, quota, and suspension status in cPanel's Email Accounts tool instead.
2. Clear Cookies and Try a Private Window
This sounds trivial but fixes a surprising share of "login loop" tickets. Roundcube relies on a PHP session cookie to keep you logged in between the login POST and the redirect to the inbox. A stale or corrupted cookie for that domain — often left over from before an SSL certificate renewal or a server migration — causes an endless bounce back to the login screen. Open an incognito/private window and try again before doing anything else.
3. Check the Error Log for the Real Message
A blank page almost always means PHP threw a fatal error and your PHP config is hiding it from the browser (correctly, for security). The real message is in the log. In cPanel, check:
~/logs/yourdomain.com/error_log
/usr/local/cpanel/logs/error_logOr via SSH, tail the general PHP error log while you reproduce the issue:
tail -f /usr/local/cpanel/logs/error_logLook for anything mentioning roundcube, a specific plugin name, or a PHP version mismatch (e.g. a plugin calling a function removed in PHP 8.1+). That tells you exactly what's failing instead of guessing.
4. Verify Dovecot Is Listening Locally
Roundcube connects to your mail server over IMAP just like a desktop client does, except it does it from localhost. If Dovecot crashed, was restarted mid-config, or a local firewall rule got too aggressive, Roundcube will show "Failed to connect to IMAP server" while everyone's phone still works fine externally. On a VPS with root access:
systemctl status dovecot
netstat -plnt | grep -E '143|993'If Dovecot isn't running, restart it and check its own log (/var/log/maillog or /var/log/dovecot.log) for the actual crash reason before assuming a one-off restart fixed it for good.
5. Rule Out a Broken Plugin
If the error log points to a specific plugin (common culprits: custom skins, third-party 2FA plugins, or old contact-sync add-ons), disable it. In stock cPanel Roundcube installs, plugins are managed through WHM > Manage Roundcube Webmail where you can toggle installed plugins without editing config files by hand. Disable the suspect one, reload, and confirm the page loads again.
6. Check Disk Space and File Permissions
Roundcube needs to write to its temp directory and, if it's using SQLite instead of MySQL for its internal database, needs that database file to be writable. A full disk (cPanel > Disk Usage) or a permissions change from a recent restore can silently break session writes, which shows up as the same login-loop symptom as a cookie problem. If disk usage is at or near 100%, that's your answer — free up space first, then retest.
7. Hard-Refresh After Any cPanel or PHP Update
If the page loads but looks unstyled or half-broken right after a cPanel version update or a PHP bump via MultiPHP Manager, it's frequently just a cached CSS/JS asset referencing an old file that no longer exists. A hard refresh (Ctrl+Shift+R or Cmd+Shift+R) clears this up in seconds, and it's worth trying before opening a ticket.
Prevention
- When you bump PHP versions for a domain, test webmail immediately afterward — don't assume it inherited the right settings.
- Keep Roundcube plugins to the ones you actually use. Every extra plugin is another thing that can break on a PHP or cPanel update.
- Monitor disk usage on mail-heavy accounts; a full disk breaks webmail sessions before it breaks anything else visible.
- Bookmark your error log path. Most "webmail is broken" tickets take under two minutes to diagnose once you've actually read the log instead of guessing.
Frequently Asked Questions
Why does webmail fail but Outlook still works fine for the same account?
Because Roundcube is a separate PHP application, not the mail server itself. If your desktop client connects fine over IMAP, the mailbox and password are confirmed working — the problem is isolated to Roundcube's PHP session, database, or a plugin, not your mail account.
What's the fastest way to tell if it's a browser problem or a server problem?
Open the webmail URL in a private/incognito window on a different device if possible. If it works there, the issue is a stale cookie or cached asset on the original browser. If it fails everywhere, it's server-side.
I see "Failed to connect to IMAP server" — does that mean my mail is down for everyone?
Not necessarily. It usually means Roundcube specifically can't reach Dovecot on localhost, which can be a local firewall rule or a Dovecot restart issue, while external IMAP/SMTP for mail clients keeps working. Test a mail client first to confirm the scope.
Can I just reinstall Roundcube to fix a persistent error?
Only as a last resort, and not before checking the error log. A reinstall can wipe custom plugin configuration and doesn't fix external causes like a full disk or a firewall rule. In cPanel/WHM environments, Roundcube updates are tied to cPanel version updates rather than a standalone reinstall.
Does clearing cookies log me out of my actual email account?
No. Clearing the browser cookie for the webmail domain only ends the Roundcube session. Your mailbox, its contents, and its password are untouched — you'll just need to log in again.
