You can set the preview frame rate in the Preview panel - it's likely yours was manually set to 50 from your previous usage.

Without seeing more of your system info (that seemingly no longer exists anyway, it's hard to diagnose beyond this. Guessing you may have also been using an export preset (one you made, perhaps?) that had the frame rate manually set at 50fps.
If you opted to wipe the AE Preferences when doing your reinstall, I would assume that's actually what fixed this. In the future, you can reset your preferences by holding CTRL+ALT+SHIFT / CMD+OPT+SHIFT while AE is starting, and then following the prompts. Quicker than reinstalling!