It is extremely unusual for a project manager to resign in the middle of the project, to which he just a few month ago commited so intensely. While there can be genuine personal reasons -which is very rare in this context- the likely reason is that one way or another, he believed that the project cannot be succesfully completed in the given environment and the given conditions.
A good, experienced project manager will know within weeks, if a project can be a success or not, not because of the task at hand -he has likely done a similar task many times before-, but because of of the environment the project happens to be IN.
Has he been equipped with enough power to make the necessary changes? After all, the essence of a project IS change. If the answer to that is NO, there is no way forward to complete the project succesfully. Cut the losses and move on.
During my days as project manager, I made it very clear, that I will not engage in any project, where the power structure is not clearly defined, because confrontations are inevitable during a project. It is not IF, it is WHEN.
And yes, I have walked away from projects only to see the project collapse completely or sink to such a level of despair, that following enlightenment the necessary changes were finally made and the project moved forward at a much greater cost in time and money.
No need to make a poll here, because almost anyone who works at CX knows, what the problem is, that needs fixing, before ANY succesfull project can be undertaken.
The speed at which things happen is painfully slow and many of us simply do not have the time to wait..