Is there already a solution to convert .all files to .cpr files under the new licencer system?
I tried to install Cubase SE3 to convert some files but this does not work anymore because I transfered my license from the old elicenser to the new system.
No there is currently no way without elicenser.
Your license for earlier versions will still be on the usb elicenser, there was not a full transfer . The C11 and earlier stay in elicenser and the new license applies from 12 onwards..
And you no longer own/possess the dongle, right?
Yes, I still do own the dongle. 
Have to find it. I stored it away into the original black Cubase box.
Didn’t knew It can be used for older versions.
I even stil have the old red Cubase VST dongle, but thats way too old.
Ha! It worked! Thanks for the help.