The discussion about this issue started quite a while ago (see related issues at the report below) and neither BanManager nor PermissionsEx had an answer for this. As a few times suggested maybe Essentials has a final answer for this.

BanManagement/BanManager#444

tl;dr: If you run Spigot 1.8 with Essentials(X), BanManager and PermissionsEx and try to use tab completion of usernames on a BM command an error gets thrown where Essentials also interferes.

Thanks for reading.

Have this problem.
PluginList:
http://puu.sh/lawNp/044691b22f.png

@JANEXLEBJAKOT we share lots of plugins. Maybe Protocollib is interfering somehow?

Could you please paste your Essentials config?

I've tried using /ban, /bminfo, /unban with tab completion, and they work just fine with essentials.

http://pastebin.com/ESiPegnN

Tried to reproduce a few minutes ago and failed too.
I could not perfectly replicate this issue earlier though.

Essentials config looks solid. What about a list of plugins.

I'm also running both Essentials and BanManager and I can't reproduce neither. Might be something very specific.

Closing, as tab completion has been properly implemented so this should no longer be an issue.