The channel entries call you are using to retrieve the custom field would need some more parameters to work - you need to give it some sort of channel or entry information for EE to work out what custom field value to return. Also, the syntax for field names requires them to be unbroken strings, so using the form search:custom field= will not work as custom ...
Yes private pages are easily do-able. There are numerous approaches, here's one using a template to control who can view content.
Create a template and add some conditional tags to determine if the person is logged out (show login form), or logged in (show the content)
Assuming that anyone that is logged in can see content
YOUR LOGIN ...
EE doc search works pretty well these days, and the info is all there.
Login and registration: https://docs.expressionengine.com/latest/member/index.html
Member groups: https://docs.expressionengine.com/latest/control-panel/member-manager.html#member-groups
Controlling access to templates: https://docs.expressionengine.com/latest/control-panel/template-...
Hard to tell from your question what actually is going on, but here is a suggestion for a way forward:
Download a clean copy of EE 5.4
Create a clean / new install of EE 5.4 on some test system
Copy the system / index files created in the new install
Update the copied system / index files with relevant information from your existing system / index files
It would be possible, if you don't mind doing a page reload as part of the process.
EE can find an entry with just the url_title or entry_id, so all you need to do is create a default template that is loaded in response to the url form blog/singlepage (e.g. by creating a template group blog and putting within it a template singlepage.html) and put in that ...
It looks like something may be tripping up a mod_security rule on the server, that can happen from time to time if the rules are too strict.
How to check:
Make the error happen using the field, make a note of the time
Check server logs to see if any mod_sec rules have been triggered
Disable any triggered rules
Test the field again, is mod_sec was the ...
The issue with fields not populating will be hard to diagnose without you providing more information about how the fields were defined and how you are trying to access them / include in templates.
As for the Publisher / NavEE support - wasn't too hard to find this on the BoldMinded site, which is indicated as working with EE4/5.
If this doesn't work with EE5,...
Pagination in EE works by adding a segment to the end of your URI - so my guess is that you have an issue with routing.
Try loading your template with the pagination block removed (i.e. so you get the page) and then add /P5to the end of your URI and see what you get. If you get the 500 error, then that would confirm the issue.
What you do next depends a bit ...
The best way of creating transient values within a template is through EE's built in layout variable system.
Layout variables can be defined as single values or as part of an addressable array, and can contain anything you can generate as tag output within a template.
The only wrinkle is that the variables only get instantiated when EE template processing ...
This sounds like a Mod Security issue. If your web host has strict mod_sec rules doing some things in EE can trigger them.
I'd suggest making the error happen, making a note of the it happens, then look in your server logs to see if a rule is being triggered - if that's the case you may need to disable one or two specific rules. If you can't access mod_sec ...