429 errors with ExecuteQueries at SharePoint

Greetings community. Our organisation runs SharePoint custom code applications with multiple ExecuteQueries. They executed commands neatly for a period of 6 months without issues. Lately, however, we are getting a lot of 429 errors, even though we made sure there is always a 3-second delay between each ExecuteQuery. We also implemented HTTP calls - no result. Did anyone experience a similar problem and how did you solve it?