As soon as I reinstall the add on manager all is fixed and I get no more error. Is this because of the position of the add-on manager in the DLL XML.
Possibly.
The other addon is causing this, most likely because it uses a different/wrong version of Simconnect and/or a different/wrong version of the VC++ runtimes libraries (LM suggest to use the 2013 runtimes, which is what we use), might change the loading order so, by reinstalling the Addon Manager, which will restore itself as the first one loaded, might fix this, because by loading it before other 3rd party modules that use the wrong/outdated version of the libraries, force the sim to load the correct ones, which is what we use.