@anna All files, including the lock files, can be deleted safely. You even should delete them after downloading them because they contain sensitive data and can be read by all apps.
Local *.ics file syncronization
Could you, please, implement two-way syncronization with the local ISC file?
It would be usefull for contacts backup and peer-to-peer contacts syncronization.
DAVdroid is a CalDAV/CardDAV sync adapter. Synchronization with local sources is not planned at the moment. Please do backups and contact sharing on the server side.
Requested feature could make possible serverless decentralized peer-to-peer syncronization using external tool like Syncthing, BTSync, etc.