“errors”: [ { “domain”: “global”, “reason”: “invalidArgument”,

Hello,
using the plugin in main site in a multisite network.
getting error in logs:
{ “error”: { “errors”: [ { “domain”: “global”, “reason”: “invalidArgument”, “message”: “Invalid To header” } ], “code”: 400, “message”: “Invalid To header” } }
As well I’m using this plugin in another network with the same plugins & themes with no issues.
What can cause this error?
Thank you