This was detected on Msys2 "win64-10" build failing those iconv calls. The failure seems to be specific to those ("win64" builds from the same source work / of "win64-10" builds both autotools and meson builds affected, as well as both branches where "win64-10" builds are available)
As "win64-10" is the default build, the issue counts as a regression between S3_0 -> S3_1 default Windows build.
Attached also S2_6 patch. This does not restart review time for later branches.
Pushed to master/S3_1/S3_0, S2_6 waits.
convert_string() iconv failing should not be a big issue, as then it just uses the original string as a fallback, and most of the time that string is just perfect.
What makes server console unusable in case of failing conversion is that convert_string() logs an error for every call where it fails, spamming the console with hundreds of messages.