Is there any additional output when you position the caret on the fastforce call? If yes, that will probably tell you what the problem is.

But my guess is that there is none, and in that case the problem is usually that the fastforce call diverges and eventually runs out of memory and crashes. That is usually the cause of lines in Isabelle being printed in red like this. This can have a number of reasons: excessive backtracking, a simplifier loop, etc.