Forum is losing images

[OCTAGRAM]

For some strange reason I just can't fix my topic because forum is loosing image attachments. Attachments are listed as all fine. They are fine in post editor and they are fine in user control panel. But first 3 out of 10 are always broken no matter what I do. I tried to delete troublesome images and make them less in size. No luck so far. Following direct attachment link gives error "The selected attachment does not exist anymore".

UPD. Topic is repaired. But I struggled for an hour, unable to fix.