This error appears when the printer is waiting for a print core to heat up, but the target temperature is not reached within 10 minutes. This can happen when the print cores are heating up before the print starts, or during printing.
Make sure that the Ultimaker 3 is not exposed to cold drafts, as this will heavily impact the nozzle temperature. Also ensure that the silicone cover is correctly in place, as shown in the image below:
Without the silicone cover, the airflow from the fans may cool the nozzles and trigger the error message.
The error “PrintCore in head slot [x] is taking too long to warm up” can also be a result of a communication error. If the Ultimaker 3 is in a suitable printing environment and silicone cover is positioned correctly, please save the log files immediately after rebooting the printer.
On the Ultimaker 3 menu, navigate to System > Maintenance > Diagnostics > Dump logs to USB. Make sure a USB stick is inserted into the printer to store the log files. Contact your local service provider, and they can analyze the log files to find the cause of the error message.