Arch Linux

No problem here. Entire system up to date?

Yes.

After testing a little around, I finally found the culprit:
The file ~/.config/netsurf/URLs contained the following section in its beginning:
---
106
~
localhost
1
file

/usr/share/netsurf/en/welcome.html
1
1450907107
1
---

After (re)moving the file, netsurf runs fine and the file is recreated containing 'localhost' instead of the tilde (~) and some differing numbers.
Solely replacing the tilde with 'localhost' leads to the removal of the section upon next start of netsurf.
Replacing the tilde with /home/<username> solves the problem.

So apparently, the respective parser for some reason lacks support for utf-8, which in turn is probably reasoned by a bug in either libutf8proc or its usage in netsurf.
But since the default string has changed from "~" to "localhost", I assume that netsurf's developers are well aware of this issue and don't need to be informed.

Update:
There are further problems.
Loading the attached very simple html file - which contains '<' and '>' in the 'href' attribute of an <a> tag - results in the same error as before modifying the 'URLs' file.
(Re)moving the whole ~/.config/netsurf does not change this behaviour.

Should I report this upstream?

By the way, I never had any other version than the current (3.3-1) of netsurf installed, so the changing from the tilde to 'localhost' is most likely not a reaction of its developers on problems with libutf8proc, rather some kind of a fallback value.

Thanks for bug report. Yes, please report this upstream! It's unclear if they are planning to follow this (new and maintained) version: http://julialang.org/utf8proc/ or these old versions: http://www.public-software-group.org/utf8proc-archive. I assume they want to use the one from the Julia project.

Having read the bug you reported due to netsurf not building with utf8proc 1.3.1 [0], I assume that this issue is related to that.
In the current version of the netsurf package (3.3-2) the issue I had is being worked around and would be fully solved, as soon as your bug report is solved somehow.

According to the comment of Chris Young on the bug tracker [1], we should use their own version of utf8proc.
Is there any reason not to do so?

[0] http://bugs.netsurf-browser.org/mantis/view.php?id=2403
[1] http://bugs.netsurf-browser.org/mantis/view.php?id=2403#c1132

Updated libutf8proc and netsurf. The test case (test.html) works here now. Please re-open this issue if this should still be a problem.

The updated package(s) will appear in [community] shortly.