I've upgraded our forum to the latest YAF update 2.2.4.14 today. It seems, all problems I saw in the v2.2.4.13 have been fixed.
However, now we can't write PMs. I can open the list of PMs (/forum/cp_pm), but when I try to write a new PM or reply, I get this:

When I turn off custom errors in the YAF web.config and the parent web.config in the root directory of our webserver, I get the following detailed error log:

Hope, this will help to solve the problem ASAP.

Does this happen with the Mobile theme or the regular theme? Did you update all files? Atleast with the support forum here i can not reproduce the issue

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This problem occurs in the regular desktop theme ("So Fresh, So Clean" now). Interesting, but I tried the mobile version of our forum on my Android smartphone, and the problem doesn't occur when I start to write a new PM.

As for updating forum source files, if a new update is not big, I compare the files using the sync tool in Total Commander and upload the changed files then. If an update is big enough, I even remove all forum source and bin files and re-upload all folders again even if most files are the same.

What source files/folders are responsible for the PM functionality? I can re-check them again using even deeper file contents comparison in Total Commander and upload the changed file if I find ones.

You need to update the page that causes the issue. In your case pages/pmmessage.ascx

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Oops... My fault. I had an ancient version of pmessage.ascx and rules.ascx from the same pages folder. I uploaded the new effective files to the server, and PMs on our forum started working properly immediately.
Thank you for your support!