Conforme declarado em esta página da Microsoft:
The limit of virtual address space for 32-bit editions of Windows-based applications is 2 gigabytes (GB). For Excel, this space is shared by the Excel application itself together with any add-ins that run in the same process. The size of the worksheet itself also affects the usage of virtual address space. Because Excel loads the worksheet into addressable memory, some worksheets that have a file size of less than 2 GB may still require Excel to use more than 2 GB of addressable memory.
A memória de 2 GB é compartilhada entre o Excel e seus plug-ins.
A única solução para o erro no memory é parar de usar esses plug-ins ou instalar a versão de 64 bits do Excel.