Hello Alexandru Popa,
Sorry for the inconvenience you're experiencing. It appears the issue with user management after the update is related to the Reg AcyMailing plugin, which is not a JoomShaper extension. We recommend contacting the plugin developer for a permanent solution.
In the meantime, you can temporarily disable the plugin by renaming the following folder:
JROOT/plugins/system/regacymailing
For example, rename it to:
JROOT/plugins/system/xregacymailing
This should restore user access until the plugin is updated.
Hope this helps!
Best regards