I increased the loglevel to DEBUG and there is some more information:
{"reqId":"q9B57JlhdRoU72IaDOJx","level":2,"time":"2019-05-13T04:31:03+00:00","remoteAddr":"xxx.xxx.xxx.xxx","user":"terminus","app":"core","method":"GET","url":"\/owncloud\/index.php\/apps\/files\/","message":"Login failed: 'terminus' (Remote IP: 'xxx.xxx.xxx.xxx')"} {"reqId":"q9B57JlhdRoU72IaDOJx","level":0,"time":"2019-05-13T04:31:03+00:00","remoteAddr":"xxx.xxx.xxx.xxx","user":"terminus","app":"OC\\User\\BasicAuthModule::auth","method":"GET","url":"\/owncloud\/index.php\/apps\/files\/","message":"Invalid password for username terminus, trying as email."} {"reqId":"q9B57JlhdRoU72IaDOJx","level":0,"time":"2019-05-13T04:31:03+00:00","remoteAddr":"xxx.xxx.xxx.xxx","user":"terminus","app":"OC\\Authentication\\Token\\DefaultTokenProvider::invalidateToken","method":"GET","url":"\/owncloud\/index.php\/apps\/files\/","message":"invalidating token 1d636250d80add81c89f2453e18a3c2f2fec42ae03a1bd23e37fac3dd6ea0f0d1f9ab051cb5d587056183dc05326f6deb365d7c8a13d1f1a6eda0b983a5547b9"} {"reqId":"q9B57JlhdRoU72IaDOJx","level":0,"time":"2019-05-13T04:31:05+00:00","remoteAddr":"xxx.xxx.xxx.xxx","user":"--","app":"OC\\Authentication\\Token\\DefaultTokenProvider::getToken","method":"GET","url":"\/owncloud\/index.php\/apps\/files\/","message":"token 1d636250d80add81c89f2453e18a3c2f2fec42ae03a1bd23e37fac3dd6ea0f0d1f9ab051cb5d587056183dc05326f6deb365d7c8a13d1f1a6eda0b983a5547b9 does not exist"} {"reqId":"q9B57JlhdRoU72IaDOJx","level":0,"time":"2019-05-13T04:31:05+00:00","remoteAddr":"xxx.xxx.xxx.xxx","user":"--","app":"OC\\Authentication\\Token\\DefaultTokenProvider::getToken","method":"GET","url":"\/owncloud\/index.php\/apps\/files\/","message":"token 9cb26d53af33efd928da1262d75fe57bb75ebac3361091e44051843ad5e7a9e1b5802d41ce29216efb3ed81a8b9609e498a68ed8a20e2b8bd0a41ab1ac5baaee does not exist"} {"reqId":"q9B57JlhdRoU72IaDOJx","level":0,"time":"2019-05-13T04:31:05+00:00","remoteAddr":"xxx.xxx.xxx.xxx","user":"--","app":"OC\\User\\Session::login","method":"GET","url":"\/owncloud\/index.php\/apps\/files\/","message":"regenerating session id for uid terminus, password set"} {"reqId":"q9B57JlhdRoU72IaDOJx","level":0,"time":"2019-05-13T04:31:05+00:00","remoteAddr":"xxx.xxx.xxx.xxx","user":"--","app":"OC\\Authentication\\Token\\DefaultTokenProvider::getToken","method":"GET","url":"\/owncloud\/index.php\/apps\/files\/","message":"token 9cb26d53af33efd928da1262d75fe57bb75ebac3361091e44051843ad5e7a9e1b5802d41ce29216efb3ed81a8b9609e498a68ed8a20e2b8bd0a41ab1ac5baaee does not exist"} {"reqId":"q9B57JlhdRoU72IaDOJx","level":0,"time":"2019-05-13T04:31:05+00:00","remoteAddr":"xxx.xxx.xxx.xxx","user":"--","app":"OC\\User\\Session::validateToken","method":"GET","url":"\/owncloud\/index.php\/apps\/files\/","message":"token 9cb26d53af33efd928da1262d75fe57bb75ebac3361091e44051843ad5e7a9e1b5802d41ce29216efb3ed81a8b9609e498a68ed8a20e2b8bd0a41ab1ac5baaee, not found"} {"reqId":"q9B57JlhdRoU72IaDOJx","level":2,"time":"2019-05-13T04:31:05+00:00","remoteAddr":"xxx.xxx.xxx.xxx","user":"--","app":"core","method":"GET","url":"\/owncloud\/index.php\/apps\/files\/","message":"Login failed: 'terminus' (Remote IP: 'xxx.xxx.xxx.xxx')"} {"reqId":"q9B57JlhdRoU72IaDOJx","level":0,"time":"2019-05-13T04:31:05+00:00","remoteAddr":"xxx.xxx.xxx.xxx","user":"--","app":"no app in context","method":"GET","url":"\/owncloud\/index.php\/apps\/files\/","message":"Current user is not logged in"} {"reqId":"e1c65744-d3aa-46a2-b753-cf161776bbc8","level":0,"time":"2019-05-13T04:31:08+00:00","remoteAddr":"xxx.xxx.xxx.xxx","user":"terminus","app":"OC\\Authentication\\Token\\DefaultTokenProvider::updateToken","method":"PROPFIND","url":"\/owncloud\/remote.php\/dav\/files\/terminus\/","message":"updating token 4701, last check is now 1557692468"} {"reqId":"e1c65744-d3aa-46a2-b753-cf161776bbc8","level":0,"time":"2019-05-13T04:31:08+00:00","remoteAddr":"xxx.xxx.xxx.xxx","user":"terminus","app":"OC\\Authentication\\Token\\DefaultTokenProvider::getToken","method":"PROPFIND","url":"\/owncloud\/remote.php\/dav\/files\/terminus\/","message":"token 32ed251eb3efda5a6f46e975c1422d0b7f25adc7bcd4966ad825de8f5bb4a7ccc7cc4c564b31b29468339417825cb62bff4cfaf9e0fad9541e4c315069c39071 does not exist"}