It is the accidentals that's the trouble, but for once Fionnnuala is wrong (sorry). It would be kind of a bummer to use a notation system that didn't allow flats ... not so much for the sake of theory as from a music perception perspective, there's a world of difference between how you, as a listener, perceive A sharp and B flat in any musical context.
Sure, they are the same pitch in well tempered tuning so they are freely interchangeable performance wise. The LOTRO synth is equally happy playing both regardless of context. In that respect Fionnuala is right.
Flats work fine, you just need to use the correct notation for the flat; prefix with underscore (not suffix with underscore like you did). Same with your sharps. Prefix, not suffix. Example: F# in ABC notation is ^F; Bb is _B. Change that and your ABC plays.
|c/2 d/2 e g d/2 e/2 |c2 z e/2 g/2 |f c c g |e2 d c/2 d/2 |e g2 d/2 e/2 |c3 e/2 g/2 |a2 g e |d2 z C/2 D/2 |E G E D |C2 z E/2 G/2 |A c B G |E2 D C/2 D/2 |E G E D |C2 z E/2 G/2 |A2 A G/2 E/2 |D4 |C/2 D/2 E B,2 |A,2 z/2 E/2 A/2 B/2 |c2 d/4 c/4 B/2 G |E2 D C/2 D/2 |E2 B,2 |A,2 z/2 E/2 A/2 B/2 |c2 d-d/2 c/4 d/4 |e2-e/2 c/2 A/2 c/2 |c G3 |G2 z z/2 F/4 G/4 |_A-_A/2 G/4 F/4 _E-_E/2 F/4 G/4 |F2 _E D |C2 _B,-_B,/2 B,/4 B,/4 |C3 z/2 F/4 G/4 |_A2 _B D/2 E/2 |^F A ^F E |D2 z ^F/2 A/2 |B2 z/2 d/2 ^c/2 A/2 |^F2 E D/2 E/2 |^F A ^F E |D2 z/2 D/2 ^F/2 A/2 |B2 z/2 B/2 A/2 ^F/2 |E2-E/2 ^F/2 E/2 D/2 |D4 ||