Converting files to 48k on import creating corrupted audio

Importing 44.1k files into my 48k project is creating files that are basically 100% square waves that freeze the meter on the track that they play from - and blow out my tweeters!

Help please? These files play the same in QuickTime. I have to use an outboard converter, like Sound Grinder to do conversions. This has never been an issue for me before. I had the same results on another computer with this same version of Nuendo.

Thanks,
PJ

Mac OSX 10.7.5
Nuendo 6.5.30