for code 28, verify the encoder pully is tight on shaft and does'nt spin. also, make sure the encoder is free to swivel. do not tighten the mounting screws as the belt may jump the teeth. play in the encoder is ok. If your connections are all ok, and you tried another known good encoder, the system card or VFC may need replacement.
Code 29 is also no encoder counts while drive is commanded. You need to go into INPUT tests I16 and manually turn encoder to verify it counts from 255 to zero as you turn it. Again, make sure gear is not spinning on shaft and belt is free to spring back. possible other cause are System card, VFC.