Hi
Hard to say without actually seeing it but I would suggest you increase Joomla error display to get the actual full error:
In Joomla global configuration:
- Under "System", set "Debug system" to Yes
- Under "Server", set "Gzip page compression" to "No"
- Under "Server", set "Error reporting" to "Maximum"
Clear Joomla cache, enable sh404SEF and visit one of the page causing the issue. Then note of any error message
in full, including all details that may be provided on the page.
What do I have configured wrong?
You don't have necessarily something configured wrong. It sounds more like an extension, most likely your template is interfering here.
A good test would actually be to simply temporarily switch to one of Joomla built-in template, protostar for instance and test with it.
Best regards
PS: I assume you mean Joomla 3.9.13.