Hello Michaela Janker
I think this is caused because you have updated SDL Studio 2017- but not MultiTerm. Both Studio and MultiTerm must always be up-to date and on the latest versions.
As you are unable to use the AutoUpdate feature in MultiTerm, please navigate to the following folder-
C:\ProgramData\SDL\SDL MultiTerm\MultiTerm14\Updates and double-click MultiTermUpdateClient.exe and follow the instructions.
Please let me know if this helps.