Thanks for the quick responses!
(Most) other posts in the same category did not get locked, so I don’t think that’s it. I checked the category settings and found “Auto-close topic after: … mins”. It is not enabled.
I found this as well, and it is not enabled.
I have seen this before, and indeed it looks different. As in your screenshot, it shows (topic deleted by author). In this case, the topic simply had a padlock icon next to it, but otherwise everything looked usual, and there was no sign of an edit history.
Any other ideas?
Maybe it’s best if I link the topic here: https://igraph.discourse.group/t/how-should-i-do-for-reducing-graph-object-memory-usage/909 (It’s already unlocked though.)
At the moment it’s not a big deal, as it’s only the second time I notice it, and it was easy to unlock the topic. But I’d like to avoid unnecessary frustration for our forum users, so it’d be good to understand what is happening.