Thanks Stefan, I think the "400" issue is fixed (r1853518, added to backport proposal), but two tests keep failing for in the test suite, namely test_004_post and test_500_proxy. They fail with or without my changes (trunk and 2.4.x), so I don't think it's related (mod_proxy does not seem to be reached for failing requests)..
My system is debian/openssl-1.1.1a, dunno if openssl version is related but forcing "SSLProtocol TLSv1.2" didn't help. The attached tarball contains an "output.log" (from: make test 2>&1 |tee output.log), "mod_h2-test.diff" which is the configuration changes I ran with (mainly trace6 with dumpio, and a small/unrelated ProxyPass "fix" for the trailing slash), and finally the error_log (trace6/dumpio). HTH...
