Hi Rene,
The general practice is to have one version of GA and FCS/RC in the support site for download of a product.
Starting 5.2 the release model is changed from FCS then GA to RC then GA(similar to ONTAP).
5.1 was a FCS release which got replaced by 5.2RC release.
The replacement of 5.1 by 5.2RC has nothing to do with the quality of 5.1 but just a general process being followed.
5.1 can still be downloaded by going to the bottom of software download page and selecting OCUM.
Hope this helps.
Regards
adai