The level of Depot at the time of creation matters. If you upgrade, it won't take it in account so upgrade first and create later.

Well aware thank you.

All depots were at level 2 or higher and a second attacker was still unable to join. We are trying again, so we'll see if anything has changed.

No change, still the same message stating that no slot is available for another attacker. I did also notice that when an ACS attack is initiated, there is no link on the fleet page, or the destination selection page.

Since there doesn't seem to be an answer, would it be wiser to report this as a bug? I know others on Speed have performed ACS attacks, so I'm not sure what is going on here, but am starting to become quite irritated with this problem.

i can confirm this
i think this is an error
all way we try still its say

Error!
No more slots for attackers available!
The ACS Depot of the ACS attack initiator cannot hold any more attackers!

i dont know how to do this thing anymore

It was a bug that got fixed, check bug reports.

Awesome, works great! Many thanks, much appreciated.