The client is unable to ... firewall
if you were trying to join a dungeon/fotm, then your issue is likely the one discussed here: https://forum-en.gw2archive.eu/forum/support/tech/Code-19-9-18-1616-101/page/4#post3200617
if so this has worked for me:
guest to one of your servers (if you’re like me then your guested servers have been decided for you) then leave the area you tried to enter (plains of ascalon for example if you were trying to get into ac) then return to your normal server and you should be fine. slightly ridiculous, but it’s the only way that i’ve found that works; aside from waiting it out i mean
(edited by Placebo.2405)