Edit: Withdrawing report – it seems that reset time for squadrons is at 6 AM like the supply and provisioning missions instead of 1 AM like the roulettes.
I did ask friends via Discord whether it was supposed to reset at "daily reset" time and the first replies said yes, so I posted the bug report, and then someone pointed out that it actually has a different daily reset time.
I have logged in again today and it is letting me do training courses now.
Original report text:
Date & Time: 20 August 1:25 AM at GMT+10 (daily reset being 1:00 AM)
Frequency: I have encountered this twice, but have not yet tested if it is reliably replicable
World name: Garuda
Character name: Zi'lani Timesong
Retainer name: -
NPC name: Regimen Board (if that counts as an NPC)
Monster name: -
Class/Level: -
Party or solo: Solo
In-game time: ???
Area and coordinates: Twin Adder Barracks 6,6
Housing: -
Steps:
1. Used the Regimen Board to assign three training courses to the squadron on 19 August, the third starting at 12:25 AM (35 minutes before daily reset for a one-hour training course)
2. Training complete at 1:25 AM
3. Assigned a mission to the squadron via the Serpent Squadron Commander NPC
4. Went back to the regimen board to assign a training course to the remaining squadron members but got the error "Unable to perform further training regimens until tomorrow."
If the third mission had concluded before daily reset time then it would make sense that I couldn't do any more training that day, but since the reset time has passed, I should have another three daily allowances now.