When checking the browser console, the below logs are visible:
(g_env) [SCRIPT:EXEC] Error while running Client Script "Run Conflict Detection": ReferenceError: GlideURL is not defined
(anonymous) @ js_includes_sp.jsx?v=11-0
js_includes_sp.jsx?v=11-03-2019_0756&lp= Unhandled exception in GlideAjax
Client Script - Run Conflict Detection
This script has the 'UI Type' set to 'All' which means it tries to run when using service portal (where the CAB Workbench lives).
This script contains a document object which is not supported in Service portal.
This is documented in the document below:
Please note that the above 'Client Script' is not present into fresh Madrid or later instances, however it may be present into instances upgraded from earlier releases.
In order to resolve the issue, please change the UI Type of the above Client Script to 'Desktop' so it will not run on Service Portal and the error will be no longer visible.