Last night, @Canderthal noted that any time he attempted to weld a specific grid he was working on, he immediately got kicked from the server. After investigating, I found no evidence in the logs to explain why that was happening, but could see him getting logged out every time he touched it.
As the grid in question is quite large, and as “game bugs” are not in the category of interesting gameplay constraints, I opted to use the Space Master’s admin tools to fix the problem by removing the grid and pasting it back in place. Normally, this would be something that requires notice, but as the change is small (one grid, moved about 20m in the process of fixing a game bug) and highly localized, I went ahead without giving notice. If anyone feels strongly that I should not do that, I’ll accept that; the process for sanctioning me isn’t hard.
Removing and re-pasting the grid worked, astonishingly.