View the related error in
/var/log/httpd/error_log
For our case
[Tue Apr 06 21:59:02.042476 2021] [:error] [pid 4038655:tid 140633264977664] [client x.x.x.x:63253] [client x.x.x.x] ModSecurity: Request body (Content-Length) is larger than the configured limit (13107200). [hostname "server16.tanzaniaservers.com"] [uri "/roundcube/"] [unique_id "YGyvdlvumqg4aqKTZmHJBwAALR8"], referer: https://server16.tanzaniaservers.com/roundcube/?_task=mail&_action=compose&_id=1591840458606caf6cecf03Resolution
nano /etc/imunify360-webshield/modsecurity/modsecurity.confchange to
SecRequestBodyLimit 52428800