error on line 2 at column 6: XML

Hi there, i have just switch from yoast to rankmath.
I have this error for sitemap : “error on line 2 at column 6: XML declaration allowed only at the start of the document”
I have done some research and tryed to fix it on my staging site first. Here is what i have done yet :
– removed all content from my function.php
– changed theme
– disable all plugin except RankMath and Woocommerce
– Resave and even changed permalink.
– clear cache multiple times .
– asked my host (kinsta) to add the line you recommend in nginx conf.
Now i’m out of idea … So if you got some ideas to stay on rankmath …

Thx