I just had a similar problem. Your thread helped. Thanks.
Fortunately I found this thread in 15 minutes not 15 hours.
I had to move my custom properties from the
ineffective inacessible nonkeyable location … object data (blenders terminology not mine) ( specific armature icon)
to
effective acessible keyable location … object (the one with the cube icon)
and refer in the script to an object … not an armature.
Is this situation documented anywhere with the push on documentation that I somwhere at Blender.org???
hm, maybe it is known to the developers. but I did not find anything in the documentation about this.
most of the time I regard this very forum here as THE documentation 
Had the same problem! 
you guys should get an award