@christoph.pech Let's work on the expired trial issue first and see if the project issue goes away.
What network are you running Fusion 360 on? Do you know if there is a proxy server or a potentially restrictive network component in play? It's possible that another application is interfering with the network communicators for Fusion 360 which is causing the intermittent authentication issues. Can you test adding exception for Fusion 360 in your firewalls and anti-virus software (if applicable)?
If you are experiencing the issue, it may be helpful to reset the LoginState.xml file.
Remove the LoginState.xml file and Restart Fusion 360:
Mac OS
1. (Close Fusion 360) In Terminal, type the below command and press enter
open ~/library/
2. In the Subsequent Finder window (Macintosh HD > Users > %Profile% > Library), browse to
~/Library/Application Support/Autodesk/Web Services
3. In the Web Services Directory, locate the file LoginState.xml
4. Right click on LoginState.xml and select Move to Trash
5. Restart Fusion 360
Windows
1. (Close Fusion 360) In windows explorer browse to %localappdata%\autodesk\web services
2. In the Web Services directory, right lick on the LoginState.xml file and select delete
3. Restart Fusion 360
If these suggestions do not help, please submit a copy of the diagnostic log files. These will be useful to analyze to look for potential issues or a history of what is happening. You

I hope this helps! Let me know if you have any questions.
Thanks,
Nathan Chandler
Principal Specialist