>> I cannot find any log entries related to the auth attempts?
'idp-process.log' should reveal what is happening with CAS module in Gluu Server. If required, make it DEBUG.
>> Am I understanding the regex change correctly by changing from c:regex="https:\/\/cas\.gluu\.org\/example_simple\.php" to c:regex="https:\/\/([A-Za-z0-9_-]+\.)*oru\.se(:\d+)?\/.*"?
No guarantee of that. 'cas.gluu.org/example_simple' is just an example. I have seen far complex regex.
You can compute and test your regular expression with various online regex tool like:
https://regex101.com/ or https://regexr.com/ etc.