"base64 encoded policy" errors occur on Portfolio infrequently but can be frustrating. They can be linked to a number of different causes, but we suggest checking a few things.
First, try a different browser on your machine. Sometimes certain browsers have settings that may interfere with image uploads--these are usually in the form of browser plugins. If a different browser works, check if you have any browser plugins running in the original browser. We'd suggest disabling them when uploading and then adding them back to see which one is interfering with Portfolio.
If you're still running into errors, you should check if you're behind a firewall that is limiting what info can be sent and received. Some Firewalls may prevent you from sending/receiving data without our service being whitelisted.
Also, in some cases, your machine's time zone may not be set up correctly--this can throw a lot of things off, strangely enough, so we'd suggest making sure your computer's time zone matches your local one.
If all else fails, we encourage you to reach out to our support team so we can help nail down the culprit.