I did a quick test in Pega 7.1.8 by loggin in as firstname.lastname@example.org and creating an OOB WO (General Task) and changing the floaction from other actions menu, but do not see the above mentioned erros. Instead am able to easily switch between flowactions.
We have found a reason for such behaviour. The fact, that the problem appears only on some flow steps, not all, allowed us to find it.
The problem was caused by the fact, that we put FlowActionHTML_Simple section into a layout in harness. This is needed to split work area as we need. Those steps, which work fine, do not have such modificaiton.
We changed harness configuration in a way, when both pyCaseHeader and FlowActionHTML_Simple are within the same layout. Actually, we put both into a section and put a section into layout.
This workaround may not be applicable for all situation, but it works in our case. At lease for now.