Dashboard modifications are not allowed while an application is being upgraded

3 answers

After opening an PMR, support came back right away with the solution (as did Krzysztof)... unfortunately for me it was RTFM:
I failed to run the RRC Migration steps (see the Migrating Requirements Composer section of the Upgrading to version 4.0 Interactive Guide). After running the migration tool at https://hostname.example.com:port/rm/web/projects/migration and restarting WebSphere the problem was solved.

I had the customer with the same issue. You must check your jts,ccm,qm and rm.log. We had following error message:
ERROR ronting.server.services.discov
ery.DiscoveryService - Server Error: Entry [5eac0b15f37d1132]
com.ibm.rdm.fronting.server.ex
ception.ServiceUnavailableExce
ption: Server state invalid: REQUIRES_MIGRATION
After finishing migration of RRC the issue has been fixed.
ERROR ronting.server.services.discov
com.ibm.rdm.fronting.server.ex
After finishing migration of RRC the issue has been fixed.

This sounds like it could be a defect. If you haven't already, you should consider filing a workitem against Web UI -> Dashboards.