This is an error that users report sometimes but we have never been able to replicate it. It is likely due to specific browser issues and stems from the fact that the ensembl version choice is being recorded as null ('ensembl' => null).
Previous users have reported that cntrl+click on ensembl version choice have helped solve the problem. Or loading parameters from a previous job which worked and then modifying them.
Let me know if either of these solutions work for you.