Re: circular outline
Awhile back, I submitted a circular PCB project to Oshpark (attached). Outline was with layer 20 (dimension). Fill was done with a typical polygon. It was a single-sided board, and initially that created an error, but the workaround was simple.
In any event, the outline was an unbroken circle done as geometry, not signal (as usual), and that in itself was not a problem.
If you do not want to post your file, maybe a "trial" submission to Oshpark will find the error?