Thank for the reply!
I was able to resolve the issue.
The external SSHFS volume was still registered in Cloudron, but the remote server was offline, causing the container to fail during startup. Since the mount point was stuck and not removable at first, I proceeded as follows:
sudo rmdir /mnt/volumes/...
mkdir /mnt/volumes/...
cd /mnt/volumes/...
By recreating an empty local folder at the exact path Cloudron expected, the app was finally able to start again. Once the app was running, I could remove the external volume cleanly from the Storage configuration in the Cloudron dashboard.