Emacs 25.3.1 on Fedora 26, with ecb 20170728.1221 installed.
After activating ECB with
ecb-minor-mode, any attempt to use any kind of completion gives me a completion buffer in place of the minibuffer. RET or mouse clicks get the error "Completion is not active in the minibuffer", and the completion buffer stays there no matter what I do.
There's nothing about this in the Info help as far as I can see, nor on http://ecb.sourceforge.net, and the link to the mailing list archives on there goes to a blank frame.