@eulenspiegel – Having various sites act in various way must be quite frustrating, all I can suggest to start is to note which site is working correctly with most (all?) of the other plugins active that you use and make that your “benchmark”.
Now compare *everything* between your benchmark site and the others that are not working and try to copy the benchmark site’s settings, etc onto the non-working sites and/or deactivate the plugins that do not match the benchmark site.
It may be very inconvenient but it may also provide the solution or hopefully at least hints as to what the problem(s) may be … also, please start your own topic as your issues as described really make it more unique than what the OP started with.
@digilamer – Thanks for sharing here, too!
– Cais.