Sometimes it joins fine first try, sometimes it gives the error message "Disconnected", sometimes it errors out and then after 5 reconnects it connects successfully, and sometimes it crashes my game. I am using MULTMC and some fabric mods (see paste-bin)

Crash register Air twice
Crash out of bounds
Different session with LOG

Tell me if you need more info. Note that I am able to login and play on this server perfectly fine sometimes. And even after creating this report I was able to login and play on the server seemingly randomly.

Please try with just multiconnect installed, this looks like a concurrency issue

I tried with just multiconnect alone and with fabric api. It still takes a couple tries to get in but not as many as before, It does not throw any errors however.

I think it is serverside problem. Something like DDOS protection. For example, here is my 6 minutes long attempts to join one of such servers: [Demo] (with crash on 4:42).
Sometimes I manage to join almost immediately, and sometimes not.

@Earthcomputer , it seems that I have some additional information that may be useful.

After some tests with TeslaCraft admin (many thanks to him for this) it became obvious that during the first few connections, the client does not send any packets at all:

After some connection attempts, the client starts sending the necessary data. But, as it was in my previous message, not always.
From his words: "Probably, this mod somehow does not connect correctly. We have a very strict packet filter on the server."

I hope this information will be useful in locating and detecting the problem.

Multiconnect doesn't change the handshake at all, which afaik hasn't changed since 1.6. The only change is a ping to the server to find out it's protocol version, when the protocol is set to "auto", which doesn't apply if you force the protocol to a specific version.

The only thing I can think of is that when the client pings the server for the server list screen, it might not be reporting the same version as when the client actually connects.

multiconnect has problems with packet timings when joining with 1.12.2 and lower
packets will be hold back and then sent rapidly
easily testable because if you join an testserver with anticheats like aac
aac will flag you for using like a 50.0 timer, what is obviously not the case, but because multiconnect changed the timing of a ton of packets it makes aac believe that, after around 5 second after join everything should be normal though

multiconnect has problems with packet timings when joining with 1.12.2 and lower packets will be hold back and then sent rapidly easily testable because if you join an testserver with anticheats like aac aac will flag you for using like a 50.0 timer, what is obviously not the case, but because multiconnect changed the timing of a ton of packets it makes aac believe that, after around 5 second after join everything should be normal though

Interesting, I wonder if there is a way around this when joining. Because everything is fine after I get in.

multiconnect has problems with packet timings when joining with 1.12.2 and lower packets will be hold back and then sent rapidly easily testable because if you join an testserver with anticheats like aac aac will flag you for using like a 50.0 timer, what is obviously not the case, but because multiconnect changed the timing of a ton of packets it makes aac believe that, after around 5 second after join everything should be normal though

Interesting, I wonder if there is a way around this when joining. Because everything is fine after I get in.

I don't know why this happens and i don't think there is a workaround. Probably some stupid little bug in the code