Checking through this and trying to repro, I didn’t notice or see any problems. No obvious errors in my console and converting worked seamlessly.
Talking to @dan, this error most likely is as a result of concurrent processes and so the deadlock happens. Basically, there are two processes waiting for each other and no one can progress because of the other.
This could be something like “a user posted and I tried to convert a topic at exactly the same time”.
@dan could not reproduce the issue either.
@chrispanag do you have any custom plugins installed? Those could also interfere.