Good news: all fixed! Thank you very much for your fast and helpful reply!
Bad news: my wife will be unhappy about it.

I have a strong feeling that I have the same problem as here:

viewtopic.php?p=666212#p666212

Is there a way to manually get rid of the laggy blueprint?

Single player. No Mods, just almost 300 hours of factory building. Yesterday was a normal session in the evening, setting some quality recyclers. Today - all saves are dead due to the same error.

I followed this one:
https://forums.factorio.com/viewtopic.php?t=101840
Did not help.

Log, manual save ...