Help needed - win10 screwed up file permissions, does not allow to change to read+write

I think perhaps you have altered the attributes of the folder (by accident) or something has forced Windows to Block writing to the folder. Without going into Windows 10 I'm speculating. Check the attributes and whom has permission as an administrator to allow yourself access if your user credentials is not on the list and make sure you have all user rights. NT (the file system) has a way of making your life hell at times.
 

Make sure you are not running SONAR "as administrator." A program running in that way is running under the hidden "administrator" account [the actual name of the account is Administrator] which is normally hidden. 
 
Do you own the folder? 
http://www.thewindowsclub.com/take-ownership-windows-8
 

slartabartfast
Make sure you are not running SONAR "as administrator." A program running in that way is running under the hidden "administrator" account [the actual name of the account is Administrator] which is normally hidden. 
 
Do you own the folder? 
http://www.thewindowsclub.com/take-ownership-windows-8
 

Thanks. I have already reset the ownership to my account. Unfortunately did not fix it ...

Not running Sonar as admin, either.

This happened to me a while back, also with one particular project. If you Google generally about problems with saving and read-only files/folders in Win10, you'll find many unresolved reports of issues with this in various apps. Like the OP, I ultimately resolved it by saving to a new location.
 
As an aside, my work PC has a new behavior of late that the Save As dialog frequently crashes the app you're saving from, even for basic stuff like Notepad. If the app doesn't have an auto-save function the work is lost.
 
For me, Win10 is the most irritating OS in DOS/Windows history (I skipped Vista).

Rob[atSound-Rehab]
Thanks. I have already reset the ownership to my account. Unfortunately did not fix it ...

Have you checked effective rights or just "set the ownership"?
 
(I have German Windows, so labels can have different spelling).
In the "Advanced security" dialog for a file/directory, on top you have "Name", and "Owned" and then tabs:
"Rights", "Watchers (not sure how to translate)", "Effective access".
In the last tab, "Effective access", you can set the "user" (set to yourself, something like "CompName\MyName", so the same as for the "Owner") and check what this user is allowed, after application of all rules. It also show why something is not allowed (if such limitations exists).
 

Before I gave up, I would re-name your troublesome folder. Then re-create a new save folder from within the program, then reload the affected project from the re-named old folder and save it to the new folder, or just copy the old folder contents if that works.
 
Check that effective permissions is the same for all the troublesome files not just the folder, as there can be a discordance between file and folder permissions. 
 
Perhaps there is a stuck leftover inheritance issue? Nominally inherited permissions should show as grayed out users in the list, but your system is not behaving nominally. Try disabling permissions inheritance in the troublesome folder. Alternatively go to the top level folder (parent) for your troublesome folder, check that it has the required permissions and then try to reset the child folders to that permissions set by choosing to "Replace all child object permissions with inheritable permissions from this object." in the parent folder to see if that will reset the permissions in your troublesome folder. 
 
Or stand on one leg and scream like a chicken.