Since I somewhat lost hope to receive an answer here, I continued my research and eventually found this:
https://mypresta.eu/en/art/developer/make-gender-required-in-prestashop.html
Then, there were translations missing, which I found in Localization > Translations > Field name translations
One reason I did not find this earlier, was that I was looking for "title", but Prestashop calls this "gender" - I don't care about gender, title matters (in letters to customers, at least) ...
Yes, bellini13, it is exactly as you say, thanks for your comments. It is cheap hosting, so it does not have sufficient resources for such an import. But otherwise it is quite sufficient and I don't want to move to a different hosting just for this import. Would it not be possible to write a php script that is able to import many products even with low resources? Even if the import takes a lot of time.
I now used the Prestashop Manager (https://www.prestashopmanager.com/) for the import. 20 seconds for almost 460 products (had to do it 4-5 times before everything was right...). It even has a free trial period of 30 days, so at least this first import came free. Unfortunately the csv has a slightly different format than Prestashop standard, but that was easy enough to adjust.
wdburgdorf's post in Setting required fields in customer registration Title, Prestashop 1.6.1.11? was marked as the answer