FYI - this comes from the author of Events Organiser
So I know what is causing it. The drop-down menu is effectively changing the admin urls. I've not looked into the details as to why, but with ozh deactivated the admin venues page url is:
However, when ozh is activated clicking the venues link takes you to:
Note that the original url still works - its just the new admin menu points to this second url. The venues page works fine, however the edit venue url (which is the above appended with ?action=edit&venue=[venue-slug]) works for the first url but not the second.
As such if you activated Ozh and manually type the first url you'll find you can still edit the venues. If you actually click the link in the new admin menu it takes you to the second url, and that produces a faulty edit-venue url. Bizarre.
I've not looked into detail as to why alternate urls are being generated, but as it happens using get_edit_term_link() produces a valid edit-venue url, so in the next update I'll be using that rather than appending ?action=edit&venue=[venue-slug]. So both plug-ins will be compatible in the next update.