When upgrading a system to 9.4 some dimensions may fail to upgrade correctly.
Symptoms will be error messages during the server start up relating to duplicate dimension elements and the server consuming more and more memory until it runs out and crashes.
The offending dimensions will have been created in version 6 or 7 using dimension worksheets. Note that not all dimensions created this way are affected.
The workaround is to create an empty 9.4 server, open a renamed copy of the existing dimension worksheet (using a normal File Open, not a Dimension Worksheet Open), create a new dimension worksheet in 9.4, copy and paste the dimension layout across, then Dimension Worksheet Save. Repeat this process for each of the dimensions that were reporting duplicate elements on the server start up. Move the newly created .dim files to the server you are upgrading and start the server.
Confirmed as a bug by Cognos and reportedly fixed in 9.4 MR1.