If there is indeed some connection with a fine precipitate in the fixer than could it be connected with a cold storage temperature of the stock solution ? After all, it isn't hot in Holland or Norway at the moment ! If this was the case, then warming and thorough remixing might (depending on exactly what the precipitate is) redissolve it, or alternatively could it be filtered out ?

There might also be a correlation with 'old' emulsions versus newer designs too. Lots of ifs in this post. tomorrow I'll be using a fixer mixed from liquid on half a dozen rolls of Ilford and Hema film. I will watch what happens carefully as I have six rolls of assorted Foma to play with soon.