I use LastPass for a lot of things including storing my personal software license files. I use the “secure note” function to save the license information and attach the license file to the secure note.

I was working on something today and trying to save a license to my machine and kept getting a dialog saying, “Error opening attachment. Error C.” Nothing really specific and very confusing. I was able to save the attachment from the LastPass web site but not through the browser extension.

I ended up finding the solution in this forum post.

1. LastPass Icon > Tools > Advanced Tools > Clear Local Cache
2. LastPass Icon > Tools > Advanced Tools > Refresh Sites

After doing a clear and refresh, the attachment saved correctly. These are probably good steps to try whenever you get any sort of error with the LastPass browser extensions. Filed for future reference.