But if that doesn't truly solve things (other than maybe clearing the error but leaving you with no audio for the instrument etc.)//..

Does the DAW have a option/setting like: "Rescan plugins"?

That might solve (different DAWs can have different types of re-scan, not sure about yours.. but some allow 'drop cache and full rescan'.. or just 'quick rescan' - but begin with quick or normal.. but if it has it, after, try 'full' type of rescan.. some have 'dangerous' sounding language, like "Delete all plugin data and rescan" kinds of prompts - best to check your Mabix manual if unsure what they offer for rescanning plugins - it solves a lot of these kinds of things.).

If you saved a DAW project with an older version of Pianoteq (and a rescan didn't seem to fix it).. in some DAWs you might be able to drag-drop or load the newer version of an instrument onto the old track (which just updates the old cached-and-perhaps-unavailable old instrument, keeping MIDI and other things - it's been a long time since I used Magix products, so not sure what's current in it.).

But if none of that solves, I'd suggest contacting either Magix support, or Pianoteq support - they're super helpful.