Sven, it is most likely NOT the new WTOOLS v1.3.1 and also not Wolfmix v0.6.7 what is causing this problem.
This error happens if Nicolaudie is "working" on the Cloud Services or something "happened" to the Cloud. Normally this will be fixed within short time.
In addition you may want to reboot your Computer and (power cycle) the Wolfmix. If you still get no access, I would recommend to open a Service Ticket and provide your pics in the description https://nicolaudiegroup.com/contact
The problem must not affect everybody. I have had few Cloud issues only with my account nobody else had.
Also logout and re-login sometimes helps to fix problems Also you may try to reset WTOOLS in SETTINGS (die Einstellungen) tab (helped me once with my Sync problems).
Finally, delete WTOOLS from your computer and DOWNLOAD it AGAIN and re-install. Maybe one file was corrupted when installed and can be fixed by re-instal it.
I can confirm that this problem is NOT WTOOLS 1.3.1 and NOT Wolfmix Firmware 0.6.7.
For older versions of WTOOLS or Wolfmix firmware you may want to open a Ticket, so they may can send it to you.