Hello @SteliosS7 ,
At this time, some non-Romanized alphanumerics and other characters won't be compatible with the current event system. I've marked your account as interested in having this issue addressed with the release of the next event system. If and when our engineers have an update regarding this feature request, they'll notify you directly.
In the meantime the only workaround would be to either use imagery (if able), or to type it out using Romanized characters (A-Z, 0-9).
One other thing, if you're able to provide some examples of the text that's having issues saving / appearing, that can help our devs make sure they're covering all their bases when testing the text's compatibility in the future event builder.
If the full text has potentially sensitive information in it that you don't feel comfortable sharing publicly, feel free to reply directly to the automated @ mention email with the sample text or call our general support so we can update the existing engineer case with that text. This can also be helpful if the Greek alphabet is getting forced into unicode, which would be potentially affecting the character limit.