[wanko]

OK, so I finally was able to reproduce your result.
Thanks for your detailled explanation, this means a lot!

With this added to gamemodes.txt:

gamemodes.txt

"maps"
{
"de_eternity"
{
}
"de_park"
{
}
"de_waterfall"
{
}
...

and this added to gamemodes_server.txt:

gamemodes_server.txt

"mapgroups"
{
"mg_active"
{
"maps"
{
"de_eternity" ""
"de_park" ""
"de_waterfall" ""
}
}
}

and with the copied pictures in the 360p directory, I get it to work, see https://i.imgur.com/v8hKL9D.jpg


The quite interesting and not understood part is, once I move the (empty) map definitions from the gamemodes.txt into the gamemodes_server.txt:

gamemodes_server.txt

"mapgroups"
{
"mg_active"
{
"maps"
{
"de_eternity" ""
"de_park" ""
"de_waterfall" ""
}
}
}
"maps"
{
"de_eternity"
{
}
"de_park"
{
}
"de_waterfall"
{
}
}

Then it no longer works, the preview pictures are gone, see https://i.imgur.com/DZctb5d.jpg

So there is a difference between gamemodes.txt and gamemodes_server.txt.
I'm just not quite sure what to make with this piece of information now.