I think the current Q & ER extension v1018 works on the principal of a 15 second delay after 5 connections. Connections being a 1.5 second intervals. Approximately 15 connections per minute. It looks like Alpha Vantage is only going to allow 5 connections per minute for free, hence the errors you are seeing.
I am a UK investor and have found that Alpha Vantage provide very poor support for UK/Irish/Luxenberg etc, domiciled funds. I have removed the Q & ER extension as I was also getting inaccurate data messing up the price history for some of the securities it could find data on. A major problem being that on some funds the prices keep switching back and forward between Pence and Pounds
I am using a free stand-alone program that was originally developed to help out users of MS Money when Microsoft stopped supporting that product. The developer is actively supporting this software and has adapted it to work with Moneydance. More feature in the pipeline. This current build very stable and the quotes very consistent. No strange prices or for UK funds switching back and forward between Pence and Pounds. It has a lot of useful features. These include obtaining quote for stocks & funds, from many stock exchanges US, Canada, UK, Europe and Australia to name but a few. The ability to convert quotes from one currency to another. Convert prices in Pence to Pounds, if required, for UK stocks and funds. Provide a means for importing all this data into Moneydance. You can also import exchange rates along with your security prices. The program is in java, like Moneydance and thus runs on Windows, Mac and Linux computers.
Alpha Vantage keeps moving the goalposts. It appears they have lowered the update frequency they allow for free connectivity and once this limit is reached Alpha Vantage will not return prices until the timer starts again. Once suspects this is a tactic to persuade users to purchased their not inexpensive packages.
The extension code will need to be altered yet again to cope with this but one wonders how often Alpha Vantage will change things or if they will perhaps just discontinue the free service.
Identifying decent sources of usable data seems to be the biggest problem.