Found the problem:
The entry I had marked in my last post
Code:
local allTrialSetsDropdownCallback = { name = subMenuEntryAllSets, filterCallback = GetFilterCallbackForSets( allSetsConstantId ) }
is missing the correct "subTable" so that the indices are correct:
Lua Code:
local allTrialSetsDropdownCallback = { { name = subMenuEntryAllSets, filterCallback = GetFilterCallbackForSets( allSetsConstantId ) } }
This was wrong in my filter plguin already but did not raise any error/worked as intended.