sorry for the delay,
I have no relevant restriction in robots.txt
I don’t use .htaccess to make redirect (I’m not able to), instead I use a plug in called 301 redirect, I think that the plug in write rules tu .htaccess.
I’ve found a rule that I’m sure I’ve not created myself. the rule was about sitemap.xlm
I’ve deleted it a few days ago and I cannot remember exactly what was written, but I remeber that was wrong. I’ve delete the rule and now I can display the sitemap.xlm in my browser.
I suspect that that SEO plug in created a wrong redirect rule in the .htaccess and this is the reason why sitemap.xlm was not accessible