I'm soon going to require this: -
If you configure a SPNEGO filter and you use IBM Business Process Manager V8.5.0 Fix Pack 1, communication problems might occur between the Process Server and the Process Center. In particular, an problem can occur after an update or migration from another version of IBM Business Process Manager where the SPNEGO Filter has not been changed.
Bottom line, check that your SPNEGO filters reflect both /ProcessCenterInternal and /ProcessServerInternal context roots.