If you're having trouble logging in, clear your cookies.
If the bottom of the page tells you "BE: 0.18.5" then you need to shift-refresh your page (or clear browser cache).
If your app doesn't work anymore, try to sign out and sign back in.
If it really doesn't work, it's probably because Lemmy 0.19 introduces many big changes with how things work. Apps still being maintained by their developer should work. Apps that have not been updated in the last couple of months most likely will not work.
If you're using an alternate web frontent (a.lemmy.nz, t.lemmy.nz, p.lemmy.nz, voyager.lemmy.nz) then you will most likely have to clear your cache, or at the very least log out and back in.
There's a post about this release by the developers here.
Also see the blog post about the original 0.19.0 release, which is where the major changes are.
Some key things:
There's a new scaled sort that helps posts from small communities be seen in your feed
Users can now block entire instances
Two factor authentication should no longer lock you out of your account (no recovery codes, though?)
You can now export your user data (community follows, blocklists, profile settings) and import into another server as a way of migrating accounts.
"active user" stats shown on the home page will now count users that vote as active. Previously a user needed to post or comment.
Got a bit of a fright with server errors and login failing, but clearing browser data helped on both accounts. Some hoops to avoid clearing more than needed.
If anyone is wondering, specifically:
Removing the JWT cookie on desktop solved the 'Incorrect login credentials' error for me - right click, inspect page, storage tab, select cookies option, select the cookie named "jwt" and press delete/backspace.
Clearing the site data solved the 'Server error' problem on mobile for me - go to lemmy.nz, click the lock indicating a secure connection, then click option to delete browsing data just for this site.
These are instructions for firefox/fennec but are likely to be the same or very similar for other browsers.
So far for me, I've had to remove and readd accounts for everything before it would work, nothing has just worked. But yes, once removed and readded they have all worked fine.
I've been waiting for this update for some time. As a weird person that primarily uses the lemmy default website, there are some minor bugs that have been annoying me and are now resolved. Some more to fill their place, of course!
Not sure why you can't tag me properly. I guess it's not properly supported in Voyager?
I'm posting this comment from the Voyager web front end and I can confirm, tagging users doesn't work.
Just a heads up, since I know this has nothing to do with your instance, but version 0.19.2 unhides comments and posts that I have deleted. Version 0.18.5 kept them hidden. Maybe report this to Lemmy devs (?)
Do you mean posts and comments you have created then deleted are suddenly back?
Do you have an example? If it's only marked as deleted, then I believe both you (as creator) and I (as admin) should be able to see it, even if it's currently marked as admin. I'm wanting to get a bit more context around it to help search for any existing reports.
Edit: oh, I think you mean that when you delete a comment, it would hide it unless you expanded the comment? Now it's shown by default, but only to you, it's still deleted?