I am having a problem with my Mikrotik router and Airgrid antenna. When I connect the Airgrid to any port (all of which are in bridge mode), the antenna has LAN communication for a few seconds and then it blocks, there is no flow between the Mikrotik and the Airgrid, while the Airgrid is still accessible via the wireless interface. If I turn on the antenna in any other device (laptop or mobile phone with USB-C network adapter), the Airgrid antenna works perfectly, as soon as I switch to Mikrotik the same blocks LAN traffic! What could be the cause of this?

How about giving some useful information - hardware involved, software, configutaion, etc.
With what you have told us, anything we say would be nothing more than a wild guess.

Tried with multiple versions of software and Mikrotik and Airgrid. It doesn't work on any version when I connect the LAN cable from Airgrid to Mikrotik! Tried with several different power supplies on both routers and it doesn't change anything! Also, the Mikrotik router was reset, that doesn't help either, so when I connect the LAN cable to Mikrotik, the Airgrid LAN is blocked!

So you need to do diagnosis and change relevant settings. E.g. set the bridge STP protocol to "none", or remove the port from the bridge and set a separate network on it.

Thanks man! You saved me from climbing on the roof and changing the antenna! I changed the setting as you said, bridge STP protocol to "none" and it worked immediately! Can you tell me, what did I do with that? I didn't touch any settings, it just suddenly started to block, it would work again after some time, and then it wouldn't work anymore! And it worked for over 5 years without any problems...

xSTP is family of protocols which is intended to detect loops in L2 network and break those loops by disabling some of links (and enable them back if loop breaks at some other point). Wikipedia as usual has a nice article about it.

Now, loop detection relies on certain behaviour of connected devices and if some of them misbehave in "suitable" way, loop can be mis-detected. Depending on the topology of your netweork this is likely to be happening ... specially so if disabling xSTP doesn't cause some traffic loops (which are usually quite notable).