"You already have a pending match."

Error everytime i try to invite to a group, or when someone tries to invite me, relogging does not fix this.

That happens, if you, or the one who you are trying to invite is in queue for something ( rdf/bg etc. ). It is a bug since some time now, next time leave the q or tell him to leave the q, when you want to invite.

except i have been in no que when this happens, especially after relogging, and it still persists

The most annoting bug after the "You have exited the Wintergrasp Battle"

leave group, logout, login. This has solved it for me every time so far. /reload isn't enough, but good ol /camp is.