When I tried to install the first Exchange 2013 CU1 server in coexistence with Exchange 2010 SP3 (yes I had updated all Exchange 2010 servers to SP3), I got the following message:
All Exchange 2010 servers in the organization must have Exchange 2010 Service Pack 3 or later installed. The following servers don’t meet this requirement: EdgeServerName
For more information, visit: http://technet.microsoft.com/library(EXCHG.150)/ms.exch.setupreadiness.E15E14CoexistenceMinMajorVersionRequirement.aspx
This was the Edge server, and I checked that this server actually had SP3 installed.
Since it was referringt to the Edge server I tried to create a new Edge subscription. After I was able to continue with the installation:)