Word Version Incompatibility

Good morning everyone, I've switched from Microsoft 365 Personal to Microsoft Plus 2021 Business. My question is, is there any incompatibility between the Word versions of these two services? I ask because when I try to work with a file saved as a "Microsoft Word 97-2003 Document," I get a memory error message. However, when I save the same file as a "Microsoft Word Document" and work with it, I don't get the error message.

Looking forward to your kind reply.

Fernando