Hi, I would recommend to you to change your theme with new one which is well maintained.
Link of themes :
https://community.mybb.com/mods.php?acti...=downloads
I recommend you themes: Flatty, emerald, , Square, focus, ,BootBb ,igame ,,myipb theme,,Volare responsive, this is some very well maintained themes and you will not have such issues with the recommended themes .
I would recommend also to avoid themes 2-3 years old
However, if you want to fix the current issue, if I am in your place I will do the following :
Step 1: go in admin panel - Templates & Style - templates - Default templates - member templates - member_register.
Copy everything in member_register and paste everything in empty text document on your dekstop (without changing anything in the template codes)
Step 2: go back to Templates & Style - templates - your theme templates - member templates - member_register.
Delete everything but before deleting - copy the content and save it on empty text document on your dekstop, in case you need the codes back.
Step 3: after you make sure member_register in your custom theme is empty, copy/paste the content which you have copied in step 1 and paste it there and save.
P. S probably it will change little the look of the registration page but I think it should fix your issue. However, if your theme is 2-3 years old u would have other errors and problems probably in future, because such themes I doubt is well maintained.
I just try to help, I recommend using new theme 2020, as I recommend in the link above.
Good luck.