The caching layer is usually disabled when a user is logged in for the frontend facing content (actual component and module output). So it's expected that only non-logged in users would see it.

As for the source of the issue, I'd suggest starting with validating the permissions on your server are correct.

i am using vagrant and also checked with 777 permission but no luck :)

I also have this issue and checked the folder permissions are correct. I am going to be playing with the path in the config a little as I saw some other issues that mentioned that. I am running php 7.
I do have old config lines that are memcache related from when I used memcache on older php version I will try to remove some of them.

I have tried setting a manual path in config, and also creating a new custom cache directory and changing the path to that. The line of code that gives the error is happening in a plugin onContentPrepare and is

$cache = JFactory::getCache();
$cache->clean('com_content');

Is it possible the frequency of this getting called is triggering the issue? I did not see this looping for example but I am trying to understand why that folder might be unavailable, though it is consistent so probably not a latency/write issue.

Is it possible the frequency of this getting called is triggering the issue?

Yes.
File cache cleaning does not use any file locking mechanism.

If we have 2 simulated requests and the first one is cleaning the cache directory /cache/* and the second one is creating a new cache file at /cache/com_content/xxxxxxxx.php then clean() will cause an error because it can not delete folder /cache/com_content/ because there is still a file.

Yes. File cache cleaning does not use any file locking mechanism.
If we have 2 simulated requests and the first one is cleaning the cache directory /cache/* and the second one is creating a new cache file at /cache/com_content/xxxxxxxx.php then clean() will cause an error because it can not delete folder /cache/com_content/ because there is still a file.

I have encountered this in our component, it is indeed due to locked files inside
the component cache subfolder inside /cache/com_somecomponent/somefile.php

But,
besides having 2 simultaneous requests,
it can occur also due to a module trying to clean the component cache
(i decided to remove that code cleaning as it is was not really needed)

So it is similar to report by @landed1

The line of code that gives the error is happening in a plugin onContentPrepare and is
$cache = JFactory::getCache();
$cache->clean('com_content');

Does it make sense to remove the generated warning ?

Set to "closed" on behalf of @alikon by The JTracker Application at issues.joomla.org/joomla-cms/18517

expected behaviour

Can you explain more @landed1:
I am using J3.9.16 but I don't know where that code is located

$cache = JFactory::getCache();
$cache->clean('com_content');