I had the same problem. Here’s the solution.
- Go to your php.ini file.
- Locate the extension “extension=php_mbstring.dll”. (ctrl+f might not work, just locate the long list with extensions in php.ini)
- Uncomment the extension, you do this by removing the semicolon in the same line.
- Save your php.ini file.
This probably won’t be the last time you’ll encounter something like this. As a rule of thumb, read the error carefully and make the appropriate change to your php.ini file.
Good luck!