You are hiting a known systemd "feature"; on top of it, the system might be trying to mount the remote SAMBA share before networking is operational.
Modify your fstab to to add to the mounting options
//192.168.1.1/FRITZ.NAS/WD-1600BEVExternal-01/share /srv/nas cifs credentials=/home/rlommers/.smbcredentials,rw,uid=rlommers,gid=rlommers,noauto,x-systemd.automount,_netdev 0 0
For the explanation, corrected to the new syntax by myself - Cute systemd trick of the day: auto-mounting remote shares
If you have remote drives – cifs, nfs, whatever – in /etc/fstab with
typical options, then you’ll probably find that the system will sit
there and wait for the network to come up on boot, then mount them,
before boot completes. That’s not terrible, but it’s not awesome
to make it super awesome, add two options:
Then what happens is the share gets mounted as soon as something tries to access it…but not before.
So boot runs as fast as possible, and as soon as you actually try to access the share, it gets mounted. Thanks, systemd!
Also from Arch Wiki to explain this feature - fstab
Automount with systemd
The same applies to remote filesystem mounts. If you want them to be
mounted only upon access, you will need to use the
noauto,x-systemd.automount parameters. In addition, you can use the
x-systemd.device-timeout= option to specify how long systemd should
wait for the filesystem to show up. Also, the _netdev option ensures
systemd understands that the mount is network dependent and order it
after the network is online.
Warning: Be sure to test the
fstab before rebooting with a
sudo mount -o remount -a and
sudo mount -o rw,remount /srv/nas as a erronous
fstab can give you problems upon boot.
See also, related, CIFS randomly losing connection to Windows share