Log files

Windows

log.txt is in the same folder as the telegram.exe if you are using the portable version. In case you installed Telegram from desktop.telegram.org, it's in Users/USER/AppData/Roaming/Telegram Desktop.

Linux

~/.local/share/TelegramDesktop/log.txt or ~/.TelegramDesktop/log.txt

MacOS

~/Library/Application Support/Telegram Desktop