Thank you for the post.
"a message about license conflict."
A license key conflict error message should be very specific. The conflicting computer's name and I.P. address will be named in the error.
"Then i bought a new Mac Mini and did a migration."
For upgrading or moving an existing installation, please consult Chapter 6 of the FileMaker Server 13 Getting Started Guide:
Migration assistant should be avoided when installing FileMaker Server as both the old version of the application and the old FileMaker Server user will be migrated. To remove the user:
a) Open Terminal
b) Input Command: sudo dscl . -delete /Users/fmserver
If the same license resides on two machines on the same network, one of the installations will need to be uninstalled and the computer rebooted to stop the license key conflict.
Yes, I uninstalled the 1st Filemaker Server, but had to reboot a few times on the 2nd FileMaker Server and then it works just great!