Error 500 on files following a name pattern

A week ago, some images in my website home page started returning a 500 error. More concretely, every image in the /img/cms/ folder whose name starts with an x results in a 500 error only when requested from a browser (tested with several browsers in different OSs). Files are accesible with wget and ftp. Permissions are 0644 which looks right.

Every other file in that folder can be accessed with no problems. It is a prestashop site (so Apache with PHP7), although I don't think the error is prestashop-related.

I looked into .httaccess file, but nothing looks fishy --and modification date is long before the problem appeared.

I have no idea what can be creating this problem. The x prefix makes me think this is some kind of rewriting/rule, but it is not in .httaccess. Also, if I asked for an unexisting file (e.g. /img/cms/xInvented.jpg) it correctly detects the 404 and does the right action.

Any hints? Files that I should take a look at, or server configs to review are welcomed.