You can add following key into "/configuration/appSettings" section in web.config file:
<add key="CMSForbiddenURLValues" value="\/:?\"<>|&%.'#[]+ =„“" />
it contains all characters forbidden by default so you can just remove characters you want not to be forbidden. E.g. if you want to set "+" character not to be forbidden you can change key to:
<add key="CMSForbiddenURLValues" value="\/:?\"<>|&%.'#[] =„“" />