BDSMLR turned off all old image servers - the old cdn server urls now all redirect to new servers (cdn012, cdn013, cdn101).
While this appears to solve all issues with incorrect or circular redirects that have plagued BDSMLR before, it now also appears that many image posts from the early days of BDSMLR are now truly defunct.
Thus, I've removed all image URL checks that used to successfully fix image links for old posts - what's there now from bsdmlr either works unmodified - or it is truly broken.
I've also transitioned to TamperMonkey for Firefox as GreaseMonkey appears to be dead.
This invalidates the usage of the (more advanced) fetch API as this only worked with GreaseMonkey due to it ignoring script/site context and thus side-stepping CORS rules.
This is OK for now with BDSMLR seemingly having fixed all their incorrect redirects (some were circular, ffs). Let's hope that they don't screw them up again, because there currently is now way to deal with them in a way that is compatible to both TamperMonkey and GreaseMonkey.