Hi Christian,
I'm happy to tell you what worked for me, in this case. Of course it might not work for you.
I find that if I'm sending "plain text e-mails" through gmail - either through accident or design - then delaysend doesn't always work. But, if I make sure I'm sending HTML, it works fine.
How to check. Select "more options" at the bottom RH corner of the message (for my setup), next to the trash icon. Make sure "Plain text mode" is off (i.e. no tick next to it). For me, that tends to ensure delaysend works. If its ticked - I can have issues.
Hope this helps.
Best,
Miles