Actually looks like the error is something different. We have tickets created by, and ticket comments from a colleague who left the company. His Assembla account got removed from the space.
In the Assembla web site it shows tickets created by him with the correct name and (Removed from space) afterwards. Reasonable
In the Assembla web site it shows ticket comments with no name at all. Reasonable I guess. We can at least still see the comments which is what is wanted.
Now to the problem.
When we query for ticket comments through the REST API these comments throw 500: Internal Server Error. There is no way that I can find to get these comments through the REST API. This looks to be a bug to me, my guess is that maybe there is a table join going on somewhere that is throwing an error. Clearly though the code that drives the web site can handle this though
As an aside it does appear as though exporting a backup may have the comments, and so maybe some later scripting and massaging may allow us to get to what we need, but this is clearly not an ideal solution when the REST API should work.
Maybe I will have to try direct with Customer support and see if this is a known issue.