While editing an existing api group it fails and you will see below error in catalina.out log
05/17 11:35:21.609 INFO (http-nio-37080-exec-10:) - [FormDefinition general] -- <Failures><Failure message="" name="GroupMembers" type="formfield" code="cms-groupmembers-validationURI"></Failure></Failures>
The catalina.out has the output of the /portalman/1/apis gateway API which contains a list of the APIs on the gateway. Now compare each key for the Group Members with output of /portalman/1/apis and you will see there is a missing key missing in the output.
Navigate to /sitebuilder/content/groups/ApiPackageGroup in cms and find the .xml file related to faulty group and remove the Api key which you which you figured out from above comparison