Hey @srkunze, welcome to the Nextcloud community and thanks for supporting Nextcloud snap 
Sorry to hear you are having issues with your Nextcloud snap installation. If you’ve come this far, you’re on the right track! There is a good chance someone in the community has had a similar issue and knows how to resolve it.
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you’re having integrity check failures due to that
Collabora_Online.Appimage. to resolve that issue follow the described steps in Resolve integrity check failure – “Integrity check failed…". -
please also check out our Wiki Configure CODE and Nextcloud office for Nextcloud snap · nextcloud-snap/nextcloud-snap Wiki · GitHub especially troubleshooting
Below are some links to how-to’s answering frequently asked questions and may help you help yourself;