@Benoit-NL This seems like the Mailo-mail-web bookmark from Open Store. It has not been updated since 2022, which is a very long time in web developement. I would consider this app abandoned, and try to find another solution.
I don't have a Mailo-account, but I would try things in this order:
Dekko, e-mail app from the Open Store where you can register you e-mail accounts and send and receive mails. A bit similar to how Thunderbird works, if you have tried that in the past. You would probably need to use IMAP to configure the e-mail client.
Download Mimi and uWolf. Log into your Mailo account and test these two browsers.
If they don't work, learn how to install "clicks" and get the uFirefox browser in stead.
Lastly, install Waydroid (a program that emulates an Android phone) and check your mail through a modern browser there, or the Mailo app if they have one that works on non Google phones.
