HTTP 100 Error on Final Export Step

[chlarsen]

I am running the latest DokuWiki version (Grebo) with PHP-FPM 7.2 in a FreeBSD 12 jail behind an Nginx reverse proxy. I get HTTP 100 errors that prevent the .zip file from being saved, although all permissions are correctly set. Static files are served via Nginx, the rest via PHP-FPM.
Explicitly setting the base-URl in the DokuWiki config does not help.
Any thoughts? Thanks a lot, indeed, as I would love to get this going!
Chris