It's not the cache setting that's the problem. It appears that something unusual has happened with your BOINC installation. It's hard to say exactly what the problem is because I've never seen anything exactly like this problem. For some reason, BOINC isn't able to use the slots. I can suggest several steps you can try to fix the problem. I suggest trying them in order, as each step gets more complex. Only proceed if the previous step doesn't fix the problem.
Step 1: Set "No New Work" and let the existing tasks complete, and then reboot your computer. Allow new tasks again. Has this fixed the problem? If not, proceed to step 2.
Step 2: Set "No New Work", let the existing tasks complete, and then detach from PrimeGrid and reboot your computer. Now re-attach to PrimeGrid and allow new tasks again. Has this fixed the problem? If not, proceed to step 3.
Step 3: Set "No New Work", let the existing tasks complete, detach from PrimeGrid, and completely uninstall BOINC. Manually delete the entire BOINC data directory. You may need to open windows explorer "As Administrator" (Windows) or use sudo (Linux) to delete the BOINC data directory because it's possible the problem is that the data directory has the wrong ownership or privileges. Reboot your computer. Install BOINC. Attach to PrimeGrid. Allow new work. If this fixes the problem, great. If not, I'm out of ideas.
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