It is not possible to register a domain via PHP-based OpenSRS domain plug-in with non-ASCII symbols in contact information.
The log of the corresponding PHP plugin contains the error:
[Tue Feb 13 01:47:43 2018] [pid 23514] AH01215: Non-ASCII character \xc3 not permitted in owner last_name.
A software-related issue POA-114736: "Non-ASCII character \xc3 not permitted in owner"
Contact your Technical Account Manager or PTA (firstname.lastname@example.org) to get the status of the issue.