I purchased an older, used Dymo LabelWriter 400 from eBay, installed it on my Windows 7 x64 desktop and printed a test page only to have it throw a very generic message: “Printer error”.
Go to your Dymo LabelWriter 400 printer properties, click the Ports tab and uncheck the “Enable bidirectional support” checkbox. While that got it working, I was a little annoyed at having to disable two-way communication between the printer and my computer. I dug a little deeper and found an old Dymo support article that mentions a firmware update to fix a similar issue. The link to the firmware update is broken, but I was able to find it at another Dymo site. After running the updater and power cycling the printer, I re-enabled bidirectional support and successfully printed several labels.
Get the firmware update HERE