Pentax ME Super shutter issue

I discovered a strange arc on last weekend's film - it was bright on the negative and only on some frames. Sometimes almost unrecognizable because of the picture background. Today I shot a test film with two lenses for all shutter speeds and the arc appears from 1/250 and as I expected is more enhanced at shorter exposure times. So I think this is 100% shutter defect. But what is strange to me is the shape on the negative. It's bright on the negative, so something must be blocking the light. The Pentax ME Super has vertically moving blades and I would expect some sort of horizontal or trapezoid stripe for the faulty blade(s). I have looked from the back and front of the shutter, but nothing wrong can be seen. Does anyone know how such a strange arc can occur during exposure?

Following images are DSLR scans of negatives (the last one is not desaturated)

Is it a fiber blocking the light from hitting the film?

I continued to investigate the problem. Examined cocked and released shutter from both side - nothing. Opened shutter - nothing. Finally I took my iPhone, set the slow-mo mode 24Ofps and..... I saw it.
There is probably some debris on the edge of the front curtain....
As you can see 1/2000 speed looks like the arc. Using 1/30 speed reveals "something" on the blade edge.

Interesting, thanks for the update.

I opened the shutter on "B" and because I knew the exact place where to look I found it. I used insect pin size 000 and removed the debris from blade. It looks good for now.... Need to check it with strip of the film but it's promissing

Excellent! I trust the film test will be AOK.