As indicated in today's patch release notes, F2P "players not being able to deposit beyond the 10Mil EC boundary into the Fleet Bank" was deemed a bug and was fixed
Actually the exact wording was "Resolved an issue with Fleet Banks where occasionally silver players would have problems making energy credit deposits."
Since reading that gave me no idea what they actually fixed, I decided to test my theory that what they addressed was the 10Mil Fleet Bank EC Ceiling for F2P accounts without the EC Cap Upgrade.
I tested this with my EC Cap Upgraded F2P account and a fleetmate's non-upgraded F2P account. We were both able to deposit into the fleet bank past the 15Mil EC mark. (Why 15mil? That's roughly all we had between us at the time.)
Still, here are a some of questions I can't answer at this time:
1) What if all members of the fleet are F2P and no one has purchased the EC Cap Upgrade? Does the 10Mil EC Boundary still exist for that fleet?
2) Is there a hard max cap for the Fleet Bank EC Balance? If so, what is that number? 1Bil ECs like the EC Cap Upgrade perhaps?