I problaby did not see the error message at the top. oops. what i initially did (knowing that www-data cannot write anything in /var/log) was: ‘touch /var/log/wordpress.log’ and then let www-data read and write that file. but then sucuri complained that the file already exists. as i mentioned, that error message i did not see on the page. now i did and solved it by deleting the file, creating the directory /var/log/wordpress, make www-data own it and then let sucuri write the file.
it works now. thanks for writing and indirectly solving my problem, i had already forgotten about it;)
