Flint2 (MT-6000) access error after booting the router

I have Flint2 equipment (MT-6000) with version 4.6.2 installed and I noticed that if I reboot the router when I try to access the settings, web browser, entering the access password I receive the message that it is wrong.
It's happened to me twice and it forces me to reset and reconfigure the device from scratch.
Can anyone help?

You mean the default UI, not luci, right?

Correct.
Access in the default UI

Do you have anti-virus software, preventing you to input password in non-https website?

I have anti-virus software, however it does not block the entry of passwords on http sites.
I tested this situation on the MT-3000 (Beryl) and it works perfectly, accepting the password and has the same firmware version (4.6.2).
This only happens in Flint2, which when rebooting forces me to reconfigure the equipment.

But can you ssh to the router or login luci, when this happen?

An attempt to access via ssh is refused by the equipment, even though access permissions have been given in the settings. When logging in via Luci, I can already access the settings with the password I created, however access by default to the UI continues to be refused with an indication of a wrong password or incorrect user.