Fuseki GUI 400 error on all queries

[Mikael Pesonen]

I've got strange problem with one Fuseki setup. Web GUI is generating 
somehow incorrect queries resulting error 400. Endpoint in GUI is set to 
/fuseki/ds which works on other setups. Also sparql update works, error 
occurs only on all select etc queries. Looking from browser developer 
tools, query content-type is set to application/x-www-form-urlencoded. 
Could that be the reason? There is no other info on the error, just code 
400. Fuseki is a bit old version 3.16.