App UI extensions not loading, with exception "Requested change of ADF session options is rejected"
It is related to the backend transactions (and their locks?) and when this happens - the only solution is to wait until this error disappear. It can take anywhere between 30 minutes and several hours to disappear.
One of the patterns to repeat the error seems to be Sandbox creation and publishing - after some user has created and published sandbox in environment, issue will appear.