Schema collection must contain exactly ONE schema!

We have a re-occurring error message in our AIMS log file:

<2014-10-29T06:25:59> 1900 Intranet-WebGIS Stadt Winterthur (Prod.) 10.61.140.38 Anonymous
 Error: An exception occurred in FDO component.
        Schema collection must contain exactly ONE schema!
 StackTrace:
  - MgFeatureServiceHandler.ProcessOperation() line 83 file d:\build\ims2013\build_58.2\ent\os\server\src\services\feature\FeatureServiceHandler.cpp
  - MgOpGetIdentityProperties.Execute() line 112 file d:\build\ims2013\build_58.2\ent\os\server\src\services\feature\OpGetIdentityProperties.cpp
  ...

The message appears intermittently.

A few days ago a MAP user called me to his desk, telling me that he can't generate any graphics anymore. Instead the following error message appeared (translated):

Error whilst generating graphics. Adding layer failed: layer_name: Exception in FDO-Component: Schema collection must contain exactly ONE schema!

I was a bit surprised as I had seen the message before - but only in AIMS log files. Suddenly more an more users were complaining about the same issue - no one was able to work anymore.

We have multiple MAPSYS in use and users connect to different enterprise models - so the issues was not related to a specific enterprise or display model. As everyone was affected the cause was most likely to be found in our database. We decided to restart Oracle and the issue was solved. Our DBA noticed, that Oracle had reached the maximum number of sessions just before the issue became apparent. We have now increased the value in order to avoid similar issues in the future. The error message in AIMS occasionally still appears.

Map 2013, SP2