[X] I have confirmed that I'm running the current version and the problem persists there (note the version number in fine print next to the add-on name on addons.thunderbird.net and compare it to the version number listed in the add-on details opened from the Add-ons page in Thunderbird; if you've got an old version, click the gear icon and select "Check for Updates").
[X] If I'm having an issue with date-parsing confusion, I've read the section about that in the user guide and I still think this is something different.
Prerequisites
Describe the bug
Starting from Thunderbird 128, clicking the "Send Later" button within the sending interface results in an empty popup.
I am currently able to resolve the issue by enabling the "Detach scheduling popup from compose window" option in the add-on settings.
How to reproduce
No response
Expected behavior
No response
Additional context
No response
Operating System
Linux
Operating System Version
Arch Linux (using Wayland)
Thunderbird version
128.3.0esr
Send Later start-up string
No response