I noticed that TeX has a open file limit, while using some additional bibliographies with
backrefs and so on (basically only 5 additional
newcites were possible in that document before hitting the limit).
I worked around that with migrating from
biblatex, which I've discovered then and which seems like a good solution to other bibliography related challenges as well.
Anyways, I found this 2007 open file limit discussion and I wondered if the situation changed with current TeX Live distributions. And if yes, how to increase the open file limit in LaTeX now.