Moodle php - "Invalid Sesskey" when creating an account or trying to get forgotten password

I'm using Moodle 2.7, and a few days ago some users reported to me that when they tried to generate a new account on '/moodle/login/signup.php' or tried to get a forgotten password on '/moodle/login/forgot_password.php' the "Invalid Sesskey" message error appeared. Looking for information about the error on Moodle, i found some possible reasons on 'https://docs.moodle.org/24/en/error/moodle/invalidsesskey' but, I tried all the options and I couldn't solve the issue. The application is on a production enviroment, and I can send them modifications or tell them what to change but I can't actually see or change their files. Any help would be greatly appreciated.