An
error has occurred in the application
...
This can occur when connecting a client application with a newer version than the one running 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 trace the error by running Flexi with debug logging
and reproducing the same error. The content of the debug logs will contain, for example:
INFO Launcher:186 - The client version '2018.2.1' differs from the server version '2018.2.0.2' => Jars will be downloaded from the server.