I have a similar problem, that I reported and for which I have a possible explanation and solution.

Basically, if your client sends "starting to tame" at 0 and "tame finished" at 10, and the server measures 10 too, it's alright. But if the network lags on the "starting to tame" packet, and the server receives it late, when the "tame finished" is received the server only has counted to 9 and rejects it.

Here I assume that packets don't contain time stamps, but if they do then my scenario does not match what is happening. On the other hand, cheating with time stamps would be easy, so I really hope there isn't any.

A possible solution would be to add a small, generic, "slowdown" delay applied to all abilities. For example, adding 1% to all delays (ideally, we would be able to customize it). This way, odds that when the first packet is sent slowly the entire cast fails would be reduced.