1: in tileset_read_toplevel() [../../../src/client/tilespec.c::1775]: Could not open '/fast/freeciv/rulesets/3rd/S2_6/data/RoundSkew.tilespec':
In secfile_from_input_file() [../../../src/utility/registry_ini.c:465]: secfile '/fast/freeciv/rulesets/3rd/S2_6/data/RoundSkew.tilespec' in section 'tilespec': Expected value
file "../../src/data/misc/colors.tilespec", line 1, pos 0
looking at: '[colors]'
included from file "/fast/freeciv/rulesets/3rd/S2_6/data/RoundSkew.tilespec", line 127
I've still had no opportunity to test on Windows to see if it behaves differently there, but maybe next week. Meanwhile note that above error for me comes already when launching the client, in initial validation of the tileset given as commandline parameter, before the client even reach the main menu.
I assume you've seen errors only when trying to launch an actual game (as topology compatibility is checked only when there's a selected map topology chosen to compare it against)