IW 15.5.5 PerformAfterAsyncProcessing(); formClass is not defined

With IntraWeb 15.5.4 (and .5) for an ISAPI.DLL we sometimes get a JavaScript console error (in FireFox F12) that says:

Error in processAjaxExecute when evaluating: PerformAfterAsyncProcessing(); formClass is not defined

The error can be trace to the IWBase_xxxx.js file. 

This does not happen with IntraWeb 15.4.2. Any idea? We haven't changed anything else.

Note that it looks like everything on the page itself is still working, but I want to ensure everything is fine before we deploy this version.

Thanks in advance!

Groetjes, Bob Swart