Simon Albroscheit of Walter de Gruyter GmbH & Co. KG notes:
I use YAP in MikTex 2.5. And there, one has to decide whether to use "dvips" or "pk fonts" as render method. I chose pk fonts because they are much faster. But YAP creates these fonts exactly once (when using them the first time) and saves them in *.pk files. In my case the "critical" file mt2syt.pk had already existed. What I had to do was easy: to delete the file C:\...\All Users\...\MiKTeX\2.6\fonts\pk\modeless\pctex\mtpro2\...\mt2syt.pk. Afterwards DVI had to create a new (and thus correct) mt2syt.pk file, this time using the _new_ version of mtpro2. That's it!