Hi @paradisiac – Have you check the login page response? (ref: https://developer.chrome.com/docs/devtools/network/reference/#response) And you can try to clear all cache/cookies of mobile browser to see if you still can login.

I am having same issue, but on my desktop. I use that 99 percent of the time so it is the one I need to use. Using a phone or tablet is sort of harsh as they do not have keyboards.
I get the feeling it is something about security.
I used Windscribe to login from different countries and ip numbers, no effect.
I have 10 wordpress sites and I cant log in to any on my desktop, only from other devices.
That is why I think it is tied to the device issue. Are you using Windows 10 too?
All the other devices that login easy are using android.
I did not change any security features so far on my main device.

Hey Paradisiac, just reposting from my own thread where I reported the same issue:

“By process of elimination, I went through all the programs that had been updated on my machine in the last 48 hours, disabled them one at a time, to see if that made a difference. And I managed to rectify the issue and get into the WP Dashboard on my laptop!

It looks like there was a conflict with the anti-virus software I was using. After uninstalling it, I was able to log in via WP Admin on any browser.

I need to see if reinstalling the anti-virus software will cause the conflict to reoccur, so perhaps I may need to chose a different AV solution going forward. But that seems to be the issue.

Thanks for all the help!”

Windows 10, bitdefender too? Last question, hostgator hosting?
I really found bitdefender allowed one to go to other hosting and login without problem but hostgator did not.
I am trying to resolve the whole issue and if you say all wordpress sites, they must be on the same server/hosting?
The other hosting was on godaddy and it had no problem with bitdefender running.
By the way, if bitdefender is installed again you cant get back on the sites that had issues before.

The bug is in the Bitdefender Free antivirus program, which is blocking this connection. I wrote an article about it on our blog.

https://blog.gigaserver.cz/chybne-prihlaseni-do-wordpress-admin-rozhrani-a-admin-rozhrani-admin-gigaserver-cz-pri-pouziti-antiviroveho-programu-bitdefender-free/

Dear Clients,

We are currently experiencing complications logging into the admin interface of the WordPress editorial system and also the admin interface of our hosting at https://admin.gigaserver.cz when using Bitdefender Free antivirus. We are experiencing this error with Bitdefender Free version 1.0.21.250. Login to the administration or hosting does not take place or ends with a login error. At the same time, the login password cannot be reset either. All indications are that the Bitdefender Free antivirus program is completely blocking the browser from working with cookies. It is therefore very likely that complications with logging in may also occur in other web applications that use cookies. Adding a web address to the exceptions in the antivirus program does not work. You need to turn off the Protection Shield function in the settings menu (the cog in the program on the top right) – Protection – Set Protection Shield to OFF. However, this will turn off the virus protection completely and your computer and your data may be at risk.

We have reported this error to Bitdefender technical support. We believe the bug will be fixed in the next Bitdefender Free update within a few hours. Until it is fixed, you must disable Protection Shield or uninstall Bitdefender Free completely and use another antivirus program.

Thanks for posting this info.
I had the same problem – and solved it by uninstalling Bitdefender from my laptop running Windows 10.
My website is hosted by Panthur.
I made no changes at all to my WordPress site – no update to core or plugins or themes or any edits at all – but suddenly I was getting errors.
I could not login to WordPress – the screen would just refresh.
I could instead access my WordPress admin area via cpanel/softaculous – but then I could not edit any WordPress pages or update any plugins etc – it would just refresh or spit out an error.
Found also that I got a 400 error when trying to enter (example) data on the booking form on my website.
Then found I was getting a 500 error when simply submitting a search string on the Panthur knowledgebase page.
I found that I could browse any pages I wanted to on the Panthur server – but whenever I tried to enter data into a form it misbehaved.
Tried 20+ suggestions mentioned on all other forums (clearing coookies, flushing dns, changing browsers, resetting wifi router, etc) but they did not work.
I was in regular contact with the tech people at Panthur and they eventually helped me solve it after 25 days of back and forth!
I hope my detailed description helps others when they try to solve the same frustrating problem.

Same situation here.
From my PC running the Bitdefender Free version:
-WordPress hosting with OVH; I can log in to the Admin Dashboard with no problem.
-WordPress hosting with StableHost; I cannot log in to the Admin Dashboard.
Disable Bitdefender, login works everywhere.

From what I can tell, Bitdefender Free version does not trust letsencrypt certificates.
So it replaces them with “Bitdefender Personal CA.avfree000000” to enable it to perform a man-in-the-middle scan for trouble.
It seems that some hosts have a setting that does not allow traffic that goes through such a process.
I have a feeling that this might have something to do with cPanel/WHM.
My OVH hosting is not using cPanel/WHM. It is possible that the GoDaddy hosting mentioned above is purely a “Managed WordPress” account and therefore not based on cPanel.
The StableHost is cPanel/WHM based.
I believe all of the other hosts mentioned above and cPanel/WHM based.

But – I have no way to confirm or test if this is the culprit.
Right now I only have testbed access to cPanel/WHM hosts.

And I gave up worrying about it because I found a workaround I am happy with:
Use Cloudflare.
Bitdefender doesn’t mind the Cloudflare certificate, doesn’t swap it out for its own, and the login works just fine.

Hopefully, this is an acceptable workaround for you as well.

Good luck!

David