Regardless of the "Sprint" key assignment (left shift, alt etc) I can't get Roach to canter anymore.
Tried a recommended in the Witcher forums method of mounting, saving and reloading to no avail.
I have the latest patch installed if that matters. Previously (before the latest patch) it happened too and was solved by dismounting and re-mounting. This time the problem seems to be permanent (exists even if loaded a much earlier save).

Geralt has no problem sprinting by foot or in a boat at the same time.

Any one knows how to resolve this?

Appreciate your help!
For those who had the same problem: re-setting the controls to default fixed the issue.

Mmmmh... maybe this will reveal to be a very stupid answer, but it's better to take every possible chance... sprinting on horseback is different from sprinting on the ground: to sprint on the ground you have to press the sprint button once, but to sprint with the horse you have to press it twice; if you press it only once the horse will trot instead of gallopping

There are some ares in big cities and even around Novigrad where your horse can't sprint... maybe you are in one of those areas?

Thank you OP.