[kovidgoyal]

I suspect there is some piece of malware, most likely a security program of some sort, that has decided that removing the execute permission on temp is a good idea as it prevents execution of .exe/.dll/.ttf files from temp directories. Unfortunately, the way they do it is also preventing the execution of directories (i.e. changing into a directory). This will only affect programs that change into directories in the temp folder. calibre's viewer and conversion routines do this, so it is affected.