We have a problem with our site on some devices, sometimes the site is displayed correctly and sometimes the message "your connection is not private" is displayed as you can see on the attached screenshots. This is not the problem of the absence of the ISRG Root X1 root certificate because sometimes the site works fine on these devices, knowing that the Let's encrypt and Whynopadlock sites are displayed correctly on these devices and before our site also worked fine on these devices even after Let's encrypt changed its root certificate. Apparently the problem is at our level but I do not know exactly what is it. The SSL Labs and Whynopadlock sites do not display any errors.
I have noticed this issue on devices running the following android versions:
Sometines our site works fine on these devices:
And sometimes they show the message bellow:
Can you please help me find the cause of this problem and help me fix it.