When working in Workflow 7.1 (I haven't experienced this issue in 7.5 yet), there is an occasional issue which prevents me from saving, debugging, or publishing the project after changes have been made.
In the past, being unable to figure out the issue, I was forced to lose work and close without saving.
Prior to affecting this fix, you may feel more comfortable backing up the project directory.
A way I've discovered to get around this issue (this has worked for me 100% of the time) is to "Ctrl + A" to select everything in the primary model of your workflow project, and then "Ctrl + X" to cut everything, leaving a blank canvas.
At this point, save your project. If still unable to save, move on to the next step anyway.
Now before panic sets in, "Ctrl + Z" to undo the cut, or "Ctrl + V" to paste everything back in. The save functionality should be restored at this point. Definitely remember to save AFTER undoing the cut.
If issues persist after taking this step, repeat the steps on any other models you have in your project.
Please leave a comment if you've seen this issue before and found other ways of dealing with it, if you know why it happens, or if you know of a real fix.
-andrew
Note - this has now occurred in 7.5 as well. The fix remains the same for me.