XAMPP msg error on launch

[abmcr]

I have installed the last edition (but not the beta) of XAMPP; when i launch Apache i get a error message (see the image)
.
I have changed the configration files for running apache on port 81, but the problem is no the port.
In the phpadmin home page i get another message of error:
The php_mbstring not found.....
What i do?

[abmcr]

I have installed the beta version and the error not appears. A only one question: for running the apache server on port 81 what i do? The same files of stable version? Thank you