Roundcube gives an "Internal Server Error" popup notification after logging in successfully

Symptoms

Roundcube gives an "Internal Server Error" pop-up notification after logging in successfully. You will find the following errors in the Dovecot maillog, indicating permissions problems with the Dovecot index files:

CONFIG_TEXT: Jan 01 00:00:00 server dovecot[#pid]: imap(user@email.tld)<1676052><mailID>: Error: Index is read-only, can't write-lock /home/username/mail/email.tld/user/dovecot.list.index.log

Cause

This is caused by incorrect ownership of Dovecot index data. The index files are owned by root:

[root@server /home/username/mail]cPs# stat ./email.tld/user/dovecot.list.index
  File: ./email.tld/username/dovecot.list.index
  Size: 968       	Blocks: 8          IO Block: 4096   regular file
Device: fd04h/64772d	Inode: 25337884943  Links: 1
Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2026-01-11 22:26:01.576964569 +0800
Modify: 2026-01-11 22:26:01.578964693 +0800
Change: 2026-01-11 22:26:01.578964693 +0800
 Birth: 2026-01-11 22:26:01.576964569 +0800

Resolution

Set the data to be owned by the user.

How to change ownership of a file or directory

Afterward, run the mailperm script to check for any other issues.

How to Fix Mailbox Permissions using the command line