Hello Cristian!
I have tried the “Ultimate member” plugin, on my test machine and didn’t find any problem.
Since I can not access your website, I can not help you with that.
There may be a problem with a combination of currently activated plugins on your website. In order to detect this, please temporarily deactivate all active plugins, except for the above, and check again. If the problem disappears, try to activate the plugins one by one until the problem reappears again. This way you will discover a problematic plugin (s), if any.
Please let me know if even after you have tried the above procedure, the problem is not solved.
Thanks.