Hi.
"Unsupported file format" most possibly means that file doesn't recognized as test file. Could you please show the output of next command:
# file /var/www/vhosts/<your subscription domain>/httpdocs/config.ini
For usual ini file, there should be something like "myfile.ini: ASCII text". If you see something else (not text) - may be your file contains some strange symbols or something like that. In that case, you can try to cope visible content of file using cat and past it to new file using Plesk File Manager. And then just rename old file to config_bak.ini, and new file to config.ini