@jdaviescoates
Thank you very much.
Absolutely right, that helped me.
ONLYOFFICE Docs 8.1.3 Package Version com.onlyoffice.coudronapp@1.17.2 restore from backup works. But updates then fail.
So I installed the latest version directly from the app store, changed the secret and made the corresponding changes in Nextcloud (/settings/admin/onlyoffice). So just a whole restart. You really don't lose anything.
Now it's running again. Thanks.