Peligroso Patzer: <Hidden Skillz: where did white go wrong here? black had such a great position around move 19>
<mang00neg: 12. Rc1 seems like the beginning of the wrong plan to me.>
Indeed, <12. Rc1> was not the best for White. (It is criticized in Vigorito, David, <Play the Semi-Slav>, Quality Chess ©2008, at page 206; this is Illustrative Game #38 in that book.)
Black’s <13. … Nd5> (not criticized by Vigorito, who mentions the stronger <13. … cxd4>, but without a clear endorsement), however, returned a solid advantage to White. In the subsequent play, White first began to go wrong with <17. Qb1?> (where Vigorito recommends <17. Qa4>, apparently relying on commentary by Ivanchuk, but <17. Qb3> is probably best here).
<18. Bf4?!> was also dubious. (Better was <18. Rcd1>.) Finally, <20. Nxe5> was inferior to the line with <20. Bxe5 Bxf3 21. Bc7 Qf6>, but at this point, even this line was probably not objectively good enough to hold. (Note that after <21. ... Qf6> in the previous line, unavailing would be <22. Bxf3 Qxf3 23. Rxc5> because of <23. … Nh4 >).