Did anyone tried to get Eccotemp FVI-12LP tankless water heater hooked to closed loop radiant floor system and had a success? Tank starts and heat the water whenever I open a drain, but totally dead if I close system and system gets internally pressurized. I'm sure the min requirements from the tank specs of 0.95 gpm flow and 5 psi are met there.

Some tankless heaters see high return water temps as an error condition and shut themselves down, which could be one issue.

Do you have an expansion tank on the system to limit the maximum pressure to something reasonable?

Did it generate an error code to allow troubleshooting? Tankless water heaters are not designed and not rated for space heating applications. As Dana indicated, you are likely dealing with a min/max temp, flow rate or pressure limitation designed to prevent the unit from failing.