Among API users are reports emerging of issues with so-called "pacing violations". These violations occur when requesting too much historical data in a too short time span. Or requesting the same data repeatedly. This is occurring since the last few days. TWS uses the same approach to get the historical data to present you the bar charts. So it could be that your first chart for an instrument is fine, but if you refresh the chart it doesn't respond. Or if you change the bar size settings it does not respond.