"[Project management] Save failed - this is unusual, check previous messages for more information"

Howdy, I figured out a way to isolate where the issue is and fix your files as Adobe still hasn't fixed this file destroying bug.

Right click on and “Create Smart Material” on folders, group a bunch or individual layers into folders and try to Create Smart Material from each. If it can, its ok, if it fails, there’s your issue, delete that layer and you should be able to save now. If not, repeat this process on the rest of your layers to find any other issues.

ADOBE. Give us a pop up window for GODS SAKE. If there's problems saving we need something more visible than that microscopic text in the Log. Why can't you have the software auto replace missing files with a red checker texture or something??? This is BAD. Please please please fix this. 

For those who doesn't find the Clean up button. First, you have to click in the recycle icon that is in the left side (above the sandclock). Then when it's done you go to File> Remove unused resources and it should work!

Hi ! i came here to also find a solution and what worked for me was hitting File>Remove unused recources 

hope this helps.

Thanks for the information I find the recycle icon but I don't know how to clean everything inside it to be able to save. In File there is no tool that says Clean.
I don't know if you could upload a photo to summarise it better? Because I can't figure it out and this problem is driving me crazy.
Sorry for the inconvenience and thank you very much.

Its works Thank you for ur answer

Greatly appreciated, totally sorted me out. Thanks much for answering!

Hi,

I had the same issue today and lost like an hour, it's even more frustrating that Adobe moved the old substance forum on their website and you can't find anything from the old threads.

I fiddled around with the resource manager (last button in the UI on the left hand side toolbar), if you have any outdated resources update all of them and also don't delete any resources (like images imported into the project). Check if you have and restore them from the recycle bin if you still have them. After updating the resources and ensuring no layers are using assets that have been deleted I managed to make it save.

Hope this helps.