Update fails after "FWCloud-Updater" has been updated to 2.0.0

[webadmin]

Hello,

we have installed a FWCloud setup via the install script on a clean Ubuntu 20.04. Now the update prompt was shown after login.
When updating, the updater will freeze here:

• [09:12:19] - Updating FWCloud-Updater application
• [09:12:57] - FWCloud-Updater has been updated successfully
• [09:12:57] - Updating FWCloud-API application
• [09:12:59] - Waiting for the server to apply updates

The Server only returns 500 (Internal Server Error) for requests like 3030/socket.io/?EIO=4&transport=po...

Where to start debugging the problem?

Thanks!