When I am unable to save existing project and see the same error, my quick fix is to "Save As" the project with a slightly different name, keep my recent work, delete the old project, and rename the new one.
Same issue is being discussed here as well. https://experienceleaguecommunities.adobe.com/t5/adobe-analytics-questions/unable-to-save-existing-p...
@AdrianG2022 Worst case, you could always create a new project, copy the panels into that new project, and then save that new project. Upon saving the new project, then you just delete the one causing you problems. Like @Jennifer_Dungan said, sometimes simple (although, I've decided it's often not so simple) things like ongoing maintenance can sometimes cause problems like these. So, you have to come up with out-of-the box solutions to work around them.
Hope it hasn't caused you too much stress.
You created this project or some other user shared this project with you? If it shared by other user and gave you Read only access then you cannot save the project. But if it created by you. Try clearing cache and try again. If the issue still persists log a ticket with Adobe Support.
One of the "hacks" I have found to get around these kinds of issues is to just SHARE the project.
Believe it or not, sharing a project also forces Adobe Analytics to save it just like performing the Save or Save As function. I'm not saying this is a fool-proof approach or even guaranteeing it will work, but I've had it work for me on many occasions.
Then, when it's performed the initial update upon sharing the file, I have then been able to perform a sequential Save afterward.
Best of luck in getting this figured out!
P.S. Something else to take into account these days is if the file is sitting in a Company Folder. I've learned that file ownership and folder placement can cause saving to become a lot more tricky in Adobe Analytics. In fact, I've had files that were shared with me that I was first able to edit that were sitting in a company folder that I later lost the ability to edit afterward. The only way I was able to regain access was to have it moved out of the company folder to the main projects folder, and then have the original owner add me back as an editor. Things can get a bit crazy ever since that feature was introduced, so please keep this in mind as well.
If for any reason you unexpectedly lose access to a newly created project before manually saving it, a recovery version of your project is saved on the Analysis Workspace landing page in a folder called Recovered Projects (Last 7 Days). You must restore the recovered project and manually save it to a desired location.
To restore a recovered project:
For detailed information, refer to the attached document. - Auto Save
Hope this helps.