New user creation error

just installed a clean new Akaunting instance on my webserver
when trying to create new users I get this error message, and no users are created, any idea how to solve this?
call to undefined function Symfony/Component/Mailer/Transport/Smtp/Stream/proc_open()
thx
Pascal

I have found the problem: in my PHP installation proc-open() and proc-close() had to be allowed (were excluded in standard installation)