Some valid tickers lead to panic when fetching from Yahoo Finance, even though they are valid. Observed that on exchanges outside the US. Examples are PETR4.SA and GGBR4.SA.

Maybe Alpha Vantage would be able to deal with those better? I know Yahoo has a history of making things difficult for this kind of use.

Sometimes other tickers (such as ETH-USD) also break, but that appears to be random or based on time of request.