NaN usually means the import type was a text file/pdf/ebook rather than a subtitle file.
Aeneas (auto timestamp that LingQ uses) doesn’t support Hebrew so pressing auto stamp button will just return NaN or an error.
The Lesson Editor has been largely forgotten about. I noticed that it doesn’t even support RTL language display right now. This tool could really use some love, so I’ll take a look at it over the next few days.