Open a SSH session to your server and the file /usr/local/apache/logs/error_log
if it is blocked by Mod Security
For example with tail -1000 /usr/local/apache/logs/error_log | grep ModSecurity
If it is blocked you will see a rule id which you can whitelist
Or you can login to the Panel > Goto Security > Mod Security . You will see the last 20 lines of the log.
Copy the matching rule id
and add it in Disabled Rules --> /usr/local/apache/conf/mod_sec_disabled_rules.conf (you can just click the link in the mod security section)
Add the rule id in the file SecRuleRemoveById [the rule id you found] and click save changes.