I'm getting an error "Can't link app version file" on quite a few Yafu tasks (about half of them) from one computer only out of my 4. Another project (tngrid) is reporting the error "disk limit exceeded". There is no disk space shortage (I have 20GB free), and I haven't specified a limit for disk space in BOINC. Any ideas what's going on?

This is one of the Yafu errors from http://yafu.myfirewall.org/yafu/result.php?resultid=1216923

<core_client_version>7.8.2</core_client_version>
<![CDATA[
<message>
couldn't start app: Can't link app version file
</message>
]]>

This is the machine in question: http://yafu.myfirewall.org/yafu/results.php?hostid=23255

Just started in the last few days, changes made to the machine that might be the cause are:
1) Some windows updates (including a major one).
2) A different (very old 4MB) graphics card was inserted into the machine (just for display purposes, not crunching).
3) Malwarebytes changed from the free version (run on demand) to the trial full version (realtime).

When Boinc starts a task, it sets up an own slot folder for it.
Into this folder it links the application from the project folder. This is somehow not possible. Something prevents the creation of this link (which is in fact a file).

I suggest to switch off antivirus and malware scanner the next time you start a fresh yafu task and check if it is working with disabled scanner.

Uninstalling Malwarebytes fixed the problem with all three projects. Strange as Malwarebytes didn't inform me it was blocking anything.

Malwarebytes is a fine program but use the free non-active version and scan for malware manually.

Most of the antiviruses allow to exclude some folders from scanning in their settings, to avoid reporting false positive (and blocking apps, like boinc) --> add boinc data folder in there.

Boinc apps never include malware or virus, ask Berkeley why - or better read the well documented wiki pages regarding boinc security of applications.