STP (File type not supported by Rhinoceros)

My STP file won’t open anymore. All necessary plugins are installed.
After I was unable to open the original STP file, I created a sample object, saved this one as STP but when I tried to reopen it, this test STP file did not open either.
Any ideas? (restarting etc. did not help either)

Which Rhino version are you running?
Also, is the STEP Import plug-in enabled in Rhino Options > Plug-ins?
Is the rhp file in the folder that is specified under properties in this dialog?

I am running Version 5 SR12, and the step import worked until Friday perfectly.
The STEP Import Plug-In is installed and loaded. I de- and reactivated before to make sure that it had not crashed and loaded properly. The rhp is existing at C:\Program Files\Rhinoceros 5.0 (64-bit)\Plug-ins\import_STEP.rhp

SR13 is the current version of RH5 so it would definitely not hurt to install that one and see if that helps thing.