Having just run into this issue myself, I figured I'd post for posterity.
Error: Couldn't remove old operator type definition sesi_temporary:

op/curve
Screwing up my curve sop in my case.
I found the issue was an .hda that was created in my personal preference, i.e. C:\Users\%USERNAME%\Documents\houdini16.0\otls
called OPcustom.hda
When I delete this file, the issue goes away.
I imagine this file could get created anywhere in your Houdini Environment.
It doesn't correlate to a crash file in my temp directory but I do recall a Houdini session crashing around the time the files was created.
It would explain why a reinstall will not fix the issue as it's in your preferences that doesn't get destroyed.
Hope this helps someone in the future.