While viewing your website, you may have the following two errors displayed:
It is indeed a problem of access to a file but several origins are possible. Here are some ideas.
Check that the path and name of the file to display do not contain special characters, such as accents. The system may not find the file to display due to the character encoding of the path or file name. Limit yourself to the characters A to Z, a to z, 0 to 9, the hyphen, underscore and space.
If you are using a file sync solution like Synology Drive Client, make sure the "Enable on-demand sync to save disk space " checkbox is unchecked. Indeed, this synchronization deletes the file locally (even if it remains visible) and PHP cannot suddenly access it.