Infiltration High Alert

I stole tech from NJ twice. Then this happened.

So I left, explored the map some more, raided Synedrion, then came back to raid NJ. Still same thing.

I could not steal tech from Synedrion as well.

What is this new mechanics? Cannot raid if Alert Level is High? Or was it some MOD I installed that prevents me this?

From the latest patch notes (Hypnos):

Attacking a Faction will now increase the Alert level of that Faction towards Phoenix. At High Alert, Phoenix Point can no longer attack that Faction and will have to wait a cooldown time for Alert level to go down. After an attack, the Threat level of missions against that Faction also increases by 1 level for a period of time.

So yes, this behaviour is intended and you probably only have to wait more time.

You should also see a warning on top of the haven info screen before you choose to attack them.

Edit & PS:
Mods can break many things, especially newly integrated features when the mods are not up to date.