An
error has occurred in the application
...
This can occur when connecting with a client application that has a newer version than the one installed on the server/cloud.
You can find out which version you have on the server/cloud in the Help menu -> About Flexi -> Application Details, or in the bottom-left corner of the old/new web interface.
You can track down the error by running Flexi with debug instructions
and receiving the same error. The content of the dumps will contain, for example:
INFO Launcher:186 - Die Client-Version '2018.2.1' unterscheidet sich von der Server-Version '2018.2.0.2' => Jars werden vom Server heruntergeladen.