A follow-up:
I was in my local store today in order to buy a new bottle of fixer - same brand as before. It turned out that *all* bottles in their stock had the same "coating"/precipitate as mentioned in my original post. Moreover, the shop's assistant told me they unpacked the bottles as of today. This shop I was in today is *not* from where I purchased the bottle in my original post, so it seems like a somewhat widespread phenomenon....strange I must say.