Sometimes, it considers the :line part or even the whole :line:char part as part of the filename, and fails to find the file. I haven't been able to recognize a pattern in the messages that go wrong. The way emacs parses error messages is quite obscure and full of hairy regexps. Probably some heuristic is going wrong.

Is the file:line:ch:line:ch something that's widely used? I suspect moving to file:line:ch line:ch solves this particular problem.