Runtime application error - Server error with customhandlers

My application started throwing customhandlers runtime errors out of the blue, apparently without any changes being pushed from Service Studio. Right now there are no TrueChange errors or warnings, and no 1CP errors in Service Studio when we publish. We also can't see any problems in the monitoring instrumentation for the eSpace or server, and another app on the same dev server runs fine. We tried changing the landing screen and that made no difference. Have also tried different user agents and Internet uplinks for good measure. Can find very little about such errors online. Tried raising a case but the form won't accept our activation code.