TYPO3 Core

btw. using the latest 6.1.5 version

Yes I can confirm that the path exists and has the right permissions, saving works fine 99% but sometimes this exception comes. Before the 6.1.5 update it happened a lot more often (interestingly it got worse the older the installation was). Often only clearing all caches helped. Not sure how it is now, but the exceptions happend at least once with 6.1.5 (when adding a new backend user). We have a lot of backend users that work in parallel so perhaps the problem is a race condition or something similar.

The one problem should be fixed in 6.1.x with #46205

Do you clear the localization cache?

I cleared all caches, which should clear the localization cache too (I guess). It still happens seldom but not that often as before 6.1.5 (we had 6.1.2 or so before that).

No, I didn't mean that you manually clear the localization cache. I meant to say, does your localization cache is cleared regular? Per cron or whatever? Maybe take a look at the timestamps of the directory.

not that I know of, but the Caching framework garbage collector is running with the frequency 864000 (default). Did not change anything. The files in the l10n directory however are recent (max 2h old).

And how much is done in the backend? Does this error comes, if someone clears the cache? (You can take a look into the log module and show only cache actions.
Every time the localization cache is cleared the chance exists that you get this error.

Usually the users should not have the rights to clear the caches (and the admin isnt doing it that often, only if errors occur or the DB should be dumped).
But if you know when this bug can happen, it sounds like it should be fixable (do not throw error but recreate cache or wait for it to be created).
It happens quite seldom and I guess many users are not reporting the issue if they dont loose data.

The problem may be, that the cache is in recreation while another process tries to access it.

So if that happens to you, the question would be, why it is cleared so often, that it is viewable.

But maybe your localization cache isn't cleared while this error happens, then it may be another problem or something else.

Did you check the cache recreation as I mentioned?

I checked the language cache files and the date / time was not really fitting when it happened the last time. It happens randomly and not that often. Is there any possibility to check if the language cache was cleared at that time (in the DB perhaps?)

In the administration log you can check that action (Select "Cache" as action to only view this ones).