I found a solution - deleting the cell cache and reloading. Apparently this bug has been around for months, possibly was fixed previously, but is now recurring.
Thanks. I tried making a new repair tool, but it didn't help. In fact, the exchanger now looks like it's repaired (the panel has closed up). But it's still showing as not repaired in the HUD, and I can't use it. Not sure if this is due to save/re…
Update: I don't think it's an out of memory problem. I installed another 4GB of RAM, bringing the total to 8GB, but it made no difference. I went back to the save that exhibits this problem, and it still happens.
[Edit: Not sure about the original issue, but my failure to load savegames seems to be fixed by switching to the experimental channel in steam. So the devs have fixed it...thanks! I din't find any info to suggest this, just took a punt.]