Flyspell-mode disables Greek accents

Question

When flyspell-modeturned on and I try to write in Greek, I get an error message in most cases when I try to enter an accent above a certain character:

<268632064> is undefined

The usual sequence for entering an accented character is the first type, followed by the corresponding vowel. Instead, when flyspell-modeturned on, and the previously entered character is interpreted as a call <268632064>, regardless of what may be what causes the error, and thus the next character appears without an accent.

So,

when flyspell-modeturned off λ ' ε→λέ

when flyspell-modein on, λ ΄ ε→ λεand error<268632064> is undefined

Note. I am using GNU Emacs 25.3.1 (x86_64-apple-darwin16.7.0, NS appkit-1504.83 Version 10.12.6 (Build 16G29)) in my macbook pro in mid-2015.