Possibility #1: These are not normal switches. These have some sort of active (powered) function, such as glowing handle, dimmer, motion sensor, smart switch of some kind, etc. They power themselves via the old "leak a little current through the incandescent bulb, it won't care" strategy, which does not play well with any modern bulb. The solution is visit the electrical supply and get two basic switches.
Possibility #2 (less likely IMO): There is something broken in how the circuit is wired. There are right ways and wrong ways to wire 2-ways (or as we call them in the US, 3-ways). You may want to pop covers off lamp and switches and double check everything against the "proper" diagrams you'll find on the web. In doing this, pay attention to function moreso than wire color. It doesn't matter which color you use for which function, as long as the functions are hooked up correctly.