Can't Import Contacts (.xlsx file)

I need to do a mass update of our contact list at least once a month due to hundreds of changes that have to be made (people added, people removed, emails and other info for custom fields updated, etc.). Every time I try to upload the file though, I get the error message below. This happens no matter what browser I am using to upload the file. It's an Excel file that's about 825kb. I've tried doing smaller uploads, but run into the same error message. There are too many changes to do these updates one by one and this is causing me hours of extra work. I've talked with CC tech regarding this issue, and was told they are aware of it and fixing the issue. I also don't have time sitting on the phone/chat every month with CC to try to get this resolved. Every time I do I'm just told the same thing - We're working on it. This has been going on for at least 6-8 months (probably longer) and still no fix, which is beyond frustrating. Anyone know a workaround or know when this will actually be fixed?