After opening the entrance to the Goblin Knight Stronghold in the Twilight Forest mod on the server, one player can mine blocks while the other cannot.

Can you try to restart server to see if it would fix issue?

I restarted the server earlier and conducted a solo test, during which I located the goblin knight stronghold once more and reopened its entrance. However, I'm currently unable to break any blocks, whereas only my teammates can, leaving me entirely unable to do so
The entrance is opening normally, but I am unable to mine blocks
"Still, there is an enchantment barrier."

Seems like this is long-standing Twilight Forest bug.
I can only suggest to disable TF progression by executing /gamerule tfEnforcedProgression false