I got the log and added a fix for the actor record by removing LD-signatures for that record. We don't need LD-signatures for that particular record. This is in current dev.
I don't know if it will fix the problem. It should get a little further before it dies. Ultimately, we're getting rid of LD-signatures because they are crap. That work is in progress but may not show up immediately. It's a major change I've already spent a few months on, and it's going to be another few months before it is finished.
Until this work is finished, the only thing we can do is convince the Friendica folks to only check LD-signatures if there is a reason to do so (such as a relayed/resent post/comment). And not check it just because it is there. I don't know why the signatures are broken because they worked for years and suddenly stopped - but I think it's more important to get rid of them than try and fix them. I also don't know if we're producing bad signatures or if Friendica has issues verifying them. I really can't say. So this puts us in an unfortunate state of existence.
If the situation is intolerable, perhaps we should just get rid of LD-signatures, but this will break private groups and I think circles/aspects until the new work is complete.