Slack not working when using eoip tunnel

Hello,
First of all thanks! great product!
Second, a question about websockets under eoip tunnel. It seems all is working great apart from slack and/or other services, using websockets. It fails tests

Web Socket (Message Proxy A): Failed (Socket closed unexpectedly)
Web Socket (Message Proxy B): Failed (Socket closed unexpectedly)
Web Socket (Message Proxy C): Failed (Socket closed unexpectedly)
Web Socket (Message Proxy D): Failed (Socket closed unexpectedly)
Web Socket (Flannel): Failed (Socket closed unexpectedly)
Web Socket (Flannel [No Compression]): Failed (Socket closed unexpectedly)

Using RouterOS v6.39.2
Can anyone help with this issue?
thank you
L.

Check your MTU. EoIP has a smaller MTU, and without adjusting for it, https often fails.