Failed to open stream: No such file or directory

not really @jurasjo; file_put_contents creates a file if it does not exist, so the file not being there is normal. moreover the file does indeed exist; https://suomika.pl/wp-content/cache/autoptimize/css/autoptimize_1a4b6f8cedfb55ab8678c5c41358f78c.css

only thing I can think of; if you have really busy site there could be a race condition where 2 processes try to create the same file at the same time, in which case the 1st process gets an exclusive lock on that file, preventing the 2nd one from creating the file. but in that case I would expect another warning really ..

There are at least 8-10 same warning in one time. So maybe You are right.