This error indicates to an UTF-8 issue where the license server tries to transmit accented letters (ä,ö,ü, etc.) while a returning procedure.
Enclosed you can find the servercomptranutil program which can process umlauts like (ä,ö,ü, etc.). Please copy both files into the "bin-folder" of the license server directory and execute the command below.
./servercomptranutil -new -transaction http://dem-eu-lic-01.eu.trimblecorp.net:8888/flexnet/services/ActivationService -return <FulfillmentID>