📝 (hestiacp): Add documentation for resolving webmail internal server…

… error

The documentation provides a solution to the common issue of encountering an internal server error when accessing webmail after migrating a user with multiple domains. It outlines the necessary permission changes to the Roundcube configuration files to resolve the problem, ensuring secure access to webmail post-migration. The reference to the HestiaCP forum thread offers additional context and validation for the solution.

hestiacp/webmail-internal-server-error-after-migration.md

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+# Webmail Internal Server Error after migration
+
+After backing up and restoring a user with multiple domains to a new host, and configuring the DNS to point to the correct IP address, the webmail interface displayed an internal server error. After going through HestiaCP forums, I found that this is a permissions issue.
+
+Here's the fix:
+
+Change permissions to php files in /etc/roundcube/
+
+    find /etc/roundcube/ -type f -iname "*php" -exec chmod 644 {} \;
+
+Once done, try to access webmail.
+
+Edit: As there are passwords inside config files, this should be a better approach to fix the issue:
+
+    chown -R hestiamail:hestiamail /etc/roundcube/
+    find /etc/roundcube/ -type f -iname "*php" -exec chmod 640 {} \;
+
+You should be able to access webmail after.
+
+Reference
+
+https://forum.hestiacp.com/t/webmail-internal-server-error/12073/7