After some digging around, it seems quite a few have had this problem before. And it seems dev intervention is the only way to fix it.
I'll admit, I did stop playing for a while, plus I went offline and online a few times (back in the days when switching didn't set your resources to zero).
I had a look and my old charter code (the time I went offline and back online again) was h6Esyb8I
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Another Invalid Charter Code Problem
#1 2018-07-15 13:33:16