Backup fails most of the times

I'm running bitwarden_rs in a container and the backup also. Most of the times the backup fails for some reason.

See my log:

2019-05-27 06:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-27 07:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-27 08:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-27 09:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-27 10:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-27 11:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-27 12:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-27 13:00:00 - Backup unsuccessfull
**2019-05-27 14:00:01 - Backup successfull**
Error: unable to open database file
2019-05-27 15:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-27 16:00:01 - Backup unsuccessfull
Error: unable to open database file
2019-05-27 17:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-27 18:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-27 19:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-27 20:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-27 21:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-27 22:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-27 23:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-28 00:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-28 01:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-28 02:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-28 03:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-28 04:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-28 05:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-28 06:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-28 07:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-28 08:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-28 09:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-28 10:00:00 - Backup unsuccessfull
Error: unable to open database file
2019-05-28 11:00:00 - Backup unsuccessfull
**2019-05-28 12:00:00 - Backup successfull**

Any idea why that happens? The log doesn't contain more info. The created files have a size of 0 when unsuccessfull.

As if the database is locked.