Hi,
I’m a user of Matomo for years (while it’s name ws Piwik 
)
I don’t upgrade very often.
My last version was 4.4, and I recently seen that UI was lost.
I was able to login on a very “light” login page.
I was able to update to last release v5.3.
But the UI problem is still here.
Any idea what’s wrong ?
You’re required to upgrade to latest 4.x before going to 5.x major.
Latest is 4.16.1
Some precisions needed :
When I’ve seen the problem, I was able to login (with the poor UI design).
There was a notification saying that a new release is available : v4.16.1.
I’ve downloaded a backup (PHP files and DB) before clicking on the upgrade link.
But after the upgrade, it seems that’s the v5.0.3 which was installed (maybe v4.16.1 and immediatly/automatically after v5.0.3).
Is it possible ?
Today, I’ve installed a fresh Matomo v5.0.3 with blank DB (and empty tables).
I’ve downloaded again a backup and compared folders/files to see differences.
No real differences.
I’m now trying to “link” the new fresh installation of Matomo v5.0.3 on the old DB (by inserting informations from /config/config.ini.php).
It seems to be OK.
I can login and get back informations (sites configured, stats, …).
Some more tests to do, and if OK, I think I’ll forget/delete the previous installation and keep only the new one.
Path to major release is made of all the minors.
I mean, you need to successfully update each step of minor release from 4.4 to 4.16 before going to major 5.0. I would prefer to start again doing all steps.
Although the big difference from 4.x to 5.x is adoption of Vue frontend framework and Angular abolition.