this caused due to an infected files on your server, and it copy itself in all folders and directories
Search files that look same as original files
for example:
wp-advanced-cache.php ( original one )
.wp-advanced-chahe.php ( malicious file )
unfortunately I found those files in all directories and folders (this happen to all php files not only one or two).
this happened to me and got it fixed after scanning server files and deleted them all
try and tell me if this got you covered.
I have the same issue !!! why is that?!
