[Geschlossen] Solving an error 504 on the front end for specific product translations

For several products that have Dutch, English and French translations users get an error 504 (Gateway timed out) for only the English translation of the product.
For instance:
versteckter Link gives an error 504
versteckter Link works fine
versteckter Link works fine

We're not sure when the error started occurring. Between Christmas and New Year we've updated many WP plugins, but a user reported the problem to us in the last days of January. Hence, we don't know if the problem is related to the WP plugin updates or not. We don't even know for sure the problem is related to WPML, but since it occurs for a specific translation, we suspect it it.
I can also not find any log file that should make me wiser. I can see in the "SSL/TLS access" log there is indeed an error 504 for "GET /product/chiapas/?lang=en HTTP/1.0", but no further information is given.