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This is really up to the Operating System :) We tell PHP to remove the directory using PHP's built-in rmdir() function. PHP calls the standard C library's rmdir() function. This handles execution to the Linux kernel which tells the filesystem module to delete the directory. Something between the filesystem module, the kernel, the C standard library, and PHP returned false even though the directory was removed. That's why you see this message.

I have a feeling this is a problem with Ionos because we had not seen this for over a decade until two days ago when another Ionos client reported an identical issue. It might be a kernel compilation parameter. We'll keep monitoring this. As you said, it's merely a cosmetic issue. In the worst case, we can just change the message to say that the installation folder may not have been removed.