There are a complicated number of reasons why some functions in modules fail in newer versions of DNN - and the most common reason is when 3rd party developers by pass the DNN API to achieve certain results - instead making a decision to perform certain actions using their own code.

This is especially the case for any modules that connect to the the registration system.

But having said this there was a very specific set of bugs in 5.6.1 that caused breaking issues in ALL sorts of weird places ... specifically these bugs related to new portal alias handling methods and to the compression system used internally by DNN.

Before you throw the InvenEvent and Rego modules out with the bathwater - I would strongly suggest upgrading the problem site to 5.6.2 - and once again examining the way these modules function on your site.

FWIW the dnn 5.6.1  and 5.6.2 releases were both publicly available in beta form for some time before their official releases - which does provide 3rd party developers ample time to test their systems against coming versions.

This will be even more the case with the move to 6.0.0 which will, regardless of the commitments to backward compatibility, most likely cause breaking issues with some older modules.  Let be honest here,  DNN in no way has the ability to control how module developers code their products, and there are any number of ways using the asp.net platform to achieve any specific outcome.   DNN does publish a very specific API which has remained backwards compatible for a long time - but not all modules follow it to the letter - opting to do some things their own way.  I mean for example there are at least 3 different database access methodologies currently recommended as options.  And any number of implementation processes.  While this sort of thing remains - there will occasionally be breaking issues.

Bottom line - if you do have issues - check with your module developer.

Westa