Maybe installing that latest Pianoteq version might fix something in your system..

but I'd just first check basics, which you've probably already done (check-boxes in the export dialog).. often I use "current settings" rather than "use high quality settings" (esp. if I'm already using high quality). Of course.. also check you selected "MP3" in the drop menu item.. I've overlooked that before (in my file browser expected to see mp3s later on.. but overlooked all the flac file types ahah).

Also, I once made a typo in there somewhere (Usually saving to desktop.. but changed it manually - and a few times other similar things occurred working fast).

Other than that low hanging fruit, I'd first try the newest 7.0.5 to see if it irons things out.. maybe for some Windows update, something dislocated "LAME audio encoding"? Have had Win updates alter the strangest things.

Fingers crossed. I can't think of any logical thing other than these presently. Have to zoom off - will check back some hours from now. Cheers dazric.