I am quite used to Serana not following me a few minutes, but after a few fast travels she returns to my side.
This time she seems to be lost completely. I played for hours and she did not show up anymore. I tried to aquire a new follower and it says that she still follows me.
Is there a console command (I am on PC) to let her leave my services? She should return to Fort Dawnguard then... I fear she hangs around somewhere in Skyrim and is stuck.

set playerfollowercount to <#>


By putting a zero you should be able to recruit again

Be careful with that command.
She does not run on the normal followers quest script. She uses a different one.
Armored trolls also use the different script.

It would be easier to PRID 2002B74 (if Dawnguard is loaded just after Skyrim update.esm in load order).
Then issue the command MoveTo Player

That did the trick!
Thank you very much.