There has a been a persistent, infrequent bug in the management of merged Google+ Pages for local where the owner of the page is unable to access the management of the page and receives a 500 Error from Google. Google knows about the bug but has been unable to quash it as of yet.
Kaleh, a Top Contributor in the Google for Business forum with lots of experience on the Plus side of the house had this to suggest.
Another option to try (based upon a report in a Google+ Discuss Forum topic) is accessing the following modified version of the URL:
If you have experienced this bug let me know whether this work around solves the problem.