I had this happen a few weeks ago. I don't remember all the details (of course), but it was an Apache ModSecurity rule being triggered by navigating to the admin. Some of the url words flagged a security keyword.
Getting around this involved finding the rule being triggered and removing it.
Look at ModSecurity Tools in WHM:
Find the rule id being triggered for the url you were trying to access.
Override the rule:
On Rules List > Edit Rules add in a directive to override the rule when matched, like (with the correct id instead of 123456).
:
<locationmatch "^/my-custom-ee-admin-name/index.php.*">
SecRuleRemoveById 123456
</locationmatch>
https://documentation.cpanel.net/display/ALD/ModSecurity+Tools
examples of location matching for disabling modsec rules