I have recently upgraded to v6.4.5 and am now getting regular Fortran runtime on one of my models. I had a look at stdoutgui.txt and it has this error message:

12:37:52 PM: Error: can\'t open file \'C:\\Program Files\\BOINC\\\\RebootPending.txt\' (error 2: the system cannot find the file specified.)

If that is really the path that is needed then I am not surprised that there is an error - note the double slashes.

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If you rebooted after the upgrade, then you can manually delete the file.
As it would be a BOINC error, you should also report it on the BOINC/dev web site, so that the people who wrote the code can fix it.