Unfortunately the default API limits are quite low for IG, and the error message is telling you that you have exceeded your limit.

Try to raise this setting in application.properties (same directory as the igzplugin.log)

Change
plugin.refreshMarketDataInterval=30000
to a higher value, like 45000, 60000, or even 70000.

This will reduce the number of requests per minute.

You could probably extend the quotas, but you would have to contact IG support for that, and I'm not sure it would be free anymore.