Did you find a solution to this? I am having the same issue.
Yes, I found a solution, but at the end, I didn't need to clear objects cache (i'm not sure it's possible)
All problems was, in fact, because the jar file located in deploy/jbpm/userlibs didn't match the processes (and java classes) deployed in the database.
So, just take care to copy the jar file with the classes corresponding to the new processes deployed.