It seems I have a similar problem.
On the computer with SDK, everything seems to work ok.
But when I move apx file to another computer it doesn't recognize it as AddOn. Both computers have the same version of AC.
With both compiler setup - Release and Debug - I got a message in Addon manager of AC that this is an old version of the plugin not compatible with this version of AC.
have tried debugging in two directions:
1. Missing library
I checked all warnings in Debug mode and found out that ACAP_STATD.lib is has been not linked to the current build. It seems to work now after changing compiler settings and library version (ACAP_STAT.lib for Release).
2. Missing Resources
When I tested with Check tool in AddOn Management I got the same result as Chris. -> "Undefined Release"
I thought that it might be because I don't have any info about AddOn version. So I tried Karl's solution:
https://archicad-talk.graphisoft.com/viewtopic.php?f=23&t=28755&p=148654&hilit=missing+resources#p14...
I stuck here because the compiler can't access modified grc file. So compilation stops.