If you receive the errors Name contains invalid characters and Only valid hostname characters are allows... when adding your domain name to your Rcs.is account, your domain name likely contains non-ASCII characters such accents, which is common for Internationalized Domain Names (IDN). To add your domain with non-ASCII characters, you’ll need to add a Punycode version of your domain name, as special characters aren’t directly supported by DNS. Punycode is a special encoding used to convert Unicode characters to ASCII, which is a smaller, restricted character set.

For example, if you wanted to add the domain екампле.цом to your Rcs.is account, it would need to be added as xn--80ajbsgfv.xn--l1adx, which is екампле.цом after being converted to Punycode. After being added as xn--80ajbsgfv.xn--l1adx, users will be able to enter екампле.цом in their web browser and access your site. Attempting to directly register екампле.цом as a domain name in Rcs.is would result in an error.

You can convert your domain to Punycode using a tool like PunyCoder.

Linux users can also install the idn package and convert a non-ASCII domain name to Punycode using these commands:

apt install idn
echo "екампле.цом" | idn