Here are the errors I managed to dig up from the logs around the time the Guest tried to log in initially. Note the Log level is set to ERROR. And the user_id matches the one of the Guest user.
{"level":"error","ts":1584546320.5513833,"caller":"mlog/log.go:175","msg":"You do not have the appropriate permissions","path":"/api/v4/teams/yz5mafigdt8iimjqii4ai7byxe/members","request_id":"fch1uq4w1id39k3ak46tpzxm3e","ip_addr":"10.252.213.206","user_id":"qcw9yao9ai8g3x3zoin4u4mtsc","method":"POST","err_where":"Permissions","http_code":403,"err_details":"userId=qcw9yao9ai8g3x3zoin4u4mtsc, permission=join_public_teams"}
{"level":"error","ts":1584546334.5095377,"caller":"mlog/log.go:175","msg":"You do not have the appropriate permissions","path":"/api/v4/teams/yz5mafigdt8iimjqii4ai7byxe/members","request_id":"m446mtf17fdufyn9x5uiqk565e","ip_addr":"10.252.213.206","user_id":"qcw9yao9ai8g3x3zoin4u4mtsc","method":"POST","err_where":"Permissions","http_code":403,"err_details":"userId=qcw9yao9ai8g3x3zoin4u4mtsc, permission=join_public_teams"}