LMS not starting: "control characters are not allowed" (LMS v8.1.2, 1617513319)

Server log:

[21-04-04 06:55:02.0681] main::init (389) Starting Logitech Media Server (v8.1.2, 1617513319, Sun Apr 4 08:44:02 CEST 2021) perl 5.032000 - arm-linux-gnueabihf-thread-multi-64int
[21-04-04 06:55:02.1019] YAML::XS::LoadFile (48) Warning: YAML::XS::Load Error: The problem:

control characters are not allowed

Would be grateful for any pointers to fix.