How to overcome 504 gateway timeout error when synced the customer from core PHP code to QuickBooks.
Hi there, anshu.sharma.
A typical browser's cache stores data from previous websites you've visited. However, as it collects information, it might cause display issues in QuickBooks Online. This might be the reason the 504 gateway timeout error appears when synced the customer from Core.
I suggest logging into your QBO account using a private browser to get rid of the error and see if we get the same thing. Browsing privately disables the cache from saving your data, and it's a good way to check issues caused by a full cache.
Use these keyboard shortcuts based on the browser you're using.
For Internet Explorer of Firefox, press Ctrl + Shift + P.