I did some more research on this subject, and i think i found a
possible reason for this error.
It seems that i get more errors when AdGroups contain more than +/-
3000 keywords.
When i create a new AdGroup, and fill this with Keywords, this process
goes very fast until the number of keywords reaches +/- 3000.
After the AdGroup reaches +/- 3000 keywords, the Unexpected Internal
Api Error's keep increasing dramatically.
Maybe this points you API Guys in the right direction tracking this
bug down! :)