I am having the same issue with the Boss template upgrade from the j3 version, The only thing I can find to fix it is to change the PHP back to 7.4, which is not a great idea...
I opened the template and saved and closed, I opened the j51 icon module, saved and closed, I deleted out the HTML folder and reinstalled. None fixed it.
https://kingdom-advancement.com/j4/
I changed it back to PHP 8 so you can see my error. Same code as Uwe above.

EDIT: next day, I have an identical problem with: https://morrobaypres.com/j4 Exactly as above, exactly as Uwe said, it was working until I adjusted the custom CSS, then it gives the error, in both PHP 8.0 and 8.1. Switching back to PHP 7.4 works.

Hello

May I suggest sending temporary administration access to your Joomla installation to info@joomla51.com and we will examine this issue further. For our reference please paste a link to this post in your email.

Ciaran

Hello

Thank you for reporting this issue.

I have just created an update for this template which should resolve this issue for you. For details on updating your template may I suggest the following article.. https://joomla51.com/tutorials/joomla-basics/updating-your-joomla-template

Ciaran

Thank you very much! That worked. In our case we also had to add to the template index.php file, just above the closing head tag
<link rel="stylesheet" type="text/css" href="<?php echo $this->baseurl ?>/templates/<?php echo $this->template?>/css/font-awesome.css" />
or now all the icons loaded. Might be just our case...
Thanks again!
Mike

Hello Ciaran
First of all thanks!
With us, the template now loads too! But we also have problems with the icons.
It's probably the "Load Font Awesome" toggle in the template settings that isn't working properly.
We don't have the knowledge like Mike to solve the problem correctly.
Let's create an override here, we use two template styles - do we then have to make two adjustments?

What can we do?
Greetings
Uwe

Hi Uwe,
I am sure Ciaran will update the theme but try this:
FTP to /public_html/templates/j51_boss that is inside the public_html folder then go into templates and the theme folder.
You can also use your webhosting file manager to reach that folder, or there is an extension you can add to Joomla to allow you to do that from within Joomla.

Then in the main folder for j51_boss you'll see the index.php file. Download that so you have a backup copy.
Then open it and scroll down until you see </head>
Then just above that copy and paste in this line:
<link rel="stylesheet" type="text/css" href="<?php echo $this->baseurl ?>/templates/<?php echo $this->template?>/css/font-awesome.css" />

That should fix it.
Let me know if that works, you can reply here.
Mike

Hello

We have just created a new update for the Boss template ensuring Font Awesome is loaded correctly without any amendments to the template.

Ciaran

Hi Mike
Thanks for your guidance.
I found the index.php file and also the location for where you suggest to put the code.
But now Ciaran has made a new version of the template available and this overwrites the index.php again.
So my problem is solved twice.
Thank you for sharing your solution with me!
Greetings
Uwe

Hello Ciaran
I downloaded and installed the new version of the template.
Now the Font Awesome will be loaded.
Thank you for the solution!
Greetings
Uwe