I need your help, guys! win10 somehow screwed up my file permissions i.e. it does not allow me to save certain Sonar projects because it claims the audio folder is read-only.
 
When trying to change it, there is a progress bar showing that it changes file attributes, but then comes back looking just the same as before, still not allowing to save ...
 
When checking file permissions, I can see that win10 somehow granted me only read access (despite being the file owner) although I always had full access. When trying to change that from the root directory downwards, it yells at me that it cannot access certain files/directories ...
 
Dedicated DAW, offline, UAC disabled ...
 
Really mad at this crap. How can that bloody OS keep me from working!?!?!

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.
 

Thanks. I meanwhile tried that. but no luck.
 
I reset the ownership of all files for the entire project drive to the DAW user, and I added the DAW user to the file permission list to have it explicitly listed with full permissions there (and in all sub-folders) although that was previously granted already by group access policies.
 
I can manually create a txt file in the audio folder (I could do so before reseting owner/permissions) but Sonar still claims it cannot write to the folder.
 
When right clicking on the folder to show it's properties, it always shows "Read-only (only applies to files in folders)" and I can deselect that, but it keeps coming back (and I just realized that is also the case on the machine I'm currently typing, yet I have not had Sonar refusing to save .. so don't know if this means anything) ...

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
 

Perhaps create another admin account and set the owner of the folders and files to it and then back to your original?  Maybe you want to do this and recreate your original account?
 
Just spit balling...

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.

FYI, this so far happened in only one project. Save As to a new directory fixed it, but none of the attempts to fix the original project in win10 worked ... did a number of disk and registry checks afterwards. all negative ... let's hope this does not return ...

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).

I don't like windows 10 much. Optimised for data collection and hiding things from view. If you are good with the PowerShell thingy or enable the magic folder life is so much easier.

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).