Have you tried uninstalling and reinstalling LR 5.5? If you can’t get it to work, uninstall LR 5.5 and reinstall LR 5.4. At least if you’re not using the Photographer’s bundle and LR is part of the cloud, then it is impossible to roll back, I think.
All the standalone versions of LR should be available, here:
Tried just now, no result. Still no functioning develop mode... But reverting to an old version should be kind of stupid right? I run one of the latests pieces of hardware and run latest versions of software, how can it just stop working...?
Thanks again for the fast response!
So, seems I found a solution. Went back to version 5.4. Started the 'Develop Module Tips' under 'Help' and that seemed to trigger something that made it work again. So thanks a lot for your help! Happy photo editing again!