rsync can't find files

I have a cron job, that makes a two different db dumps, and then saves all files from the website to adrive.com and also the dp dump to adrive in a 2nd rsync command

Since Feb 24 it produces this error:
rsync: rename "/home/user/dokumente/backup/.backup_mysql_autofill.sql.gz.NABXKv" -> "backup/backup_mysql_autofill.sql.gz": No such file or directory (2)
rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1196) [sender=3.1.2]

It produces the same error for thiny files (200-500 bytes)
It always makes the error for the SAME file.
It can rsync the first db dump (800MB), it can rsync the DB log file (45 MB), but always stops at the 2nd dump (770 MB)

Now here comes the joke. Rsync worked again for 2 nights this Monday and Thuesday, then the error came back yesterday night.

So, I did not have a backup from Feb 24 Until March 2, when it sudddenly worked again for March 2 and March 3 (the whole script just did the job)

HEre is my rsync command:

BACK_USR="bodomalo@xxx.com"
BACK_SVR="rsync.adrive.com"
BACK_DIR="/opt/backup"
cd $BACK_DIR
rsync -avz -e 'ssh -i /root/.ssh/adrive.id' $BACK_DIR $BACK_USR@$BACK_SVR:dokumente

Any idea?