XF 1.5 PHP Version 7.3 causing server errors, cannot log in to admin panel

[Paradoxer]

I am currently running Xenforo on what I believe is version 1.5.1. If I select PHP Version 7.3, it returns that there are server errors, and the forum is inaccessible. When a version below 7.3 is selected, things start to work properly again. Is this a known incompatibility?

A separate issue is that the administrator panel won't allow me to log in, even on PHP Version 7.0. I can access admin.php, however whenever I put in my name and password, it will act as though it is sending me to the dashboard for a few moments—with the input fields disappearing and the Xenforo logo shrinking and moving towards the top—but then everything will refresh and the page will go back to before I input the password.

 

[Alpha1]

Your xenforo version is outdated. You should upgrade to XF 1.5.23
I am running PHP 7.3 fine with xf 1.5.23. Read this:

Is XF 1.5 compatible with PHP 7.3?

Are there any deprecations or other issues with PHP 7.3? Has anyone tried yet?

xenforo.com

XenForo with PHP 7.3

Has anyone tried running the board on PHP 7.3 with stable results?

xenforo.com

XF 2.1 - PHP 7.3.1

@Chris D With the release of PHP 7.3.1 today, has any php bugs resolved and you deem it safe to run all latest XF (2.1.x included) with PHP 7.3?

xenforo.com

 

[Paradoxer]

Your xenforo version is outdated. You should upgrade to XF 1.5.23
I am running PHP 7.3 fine with xf 1.5.23. Read this:

Is XF 1.5 compatible with PHP 7.3?

Are there any deprecations or other issues with PHP 7.3? Has anyone tried yet?

xenforo.com

XenForo with PHP 7.3

Has anyone tried running the board on PHP 7.3 with stable results?

xenforo.com

XF 2.1 - PHP 7.3.1

@Chris D With the release of PHP 7.3.1 today, has any php bugs resolved and you deem it safe to run all latest XF (2.1.x included) with PHP 7.3?

xenforo.com

The latest I have for my license is 1.5.14. I can't justify purchasing a new one because the forum is defunct, but I want to keep it around for legacy purposes, and to be able to access it properly as an admin.

I tried downloading the 1.5.14 update files and transferred them to the Xenforo root folder via FileZilla. However, when I try to install it now, I get a "web server is down" error 521 and 520. When I try to access the forums on the front-end, it's the same error. When I try admin.php, it gives me: "The site is currently waiting to be upgraded. Please complete the upgrade."

I can access the install page, which properly gives me the prompt: "Click the button below to begin the upgrade to 1.5.14." But mere moments after I start, it will give me an error and not complete.

 

[Paradoxer]

I was able to complete the installation by reducing the PHP Version to 5.6, however that did not help me with the admin.php login issue. I still have the problem I described initially, even with PHP Version 5.6. Sometimes, if I try enough, it'll successfully log in to the first page, but if I try to navigate to anywhere else within the dashboard it will kick me back to the initial admin.php log in page.

I also still get the same error accessing the forums if I go to PHP version 7.3.