It looks like your log is corrupted.
Can you try to rename the current log file over FTP, then log out and log in back to WordPress, go back to NinjaFirewall > Logs and check if you see the log content (it should display that you have logged in)?
I have done that. Same error. I actually have this error in all installations 🙁
Can you download the log and open it with your text editor to check its content? It should have 12 similar fields per line, each enclosed in [ and ]:
[10 digits] [1 or more digits] [domain.com] [#+7 digits] [1 digit] [1 digit] [IP address] [3 digits] [GET/POST] [URI] [threat info] [threat detail (hex-encoded)]
[1734109528] [0.00154] [] [#7984975] [0] [1] [xxxxx] [404] [GET] [/wp-login.php] [Blocked access to the login page] [hex:asdsdadaas]
[1734109530] [0.00209] [] [#8002054] [0] [1] [xxxxx] [404] [GET] [/wp-login.php] [Blocked access to the login page] [hex:asdsdadaas]
[1734111198] [0.00155] [] [#2108671] [0] [1] [xxxxx] [404] [GET] [/wp-login.php] [Blocked access to the login page] [hex:asdsdadaas]
[1734111207] [0.00169] [] [#4204150] [0] [1] [xxxxx] [404] [GET] [/wp-login.php] [Blocked access to the login page] [hex:asdsdadaas]
[1734111210] [0.00186] [] [#1382492] [0] [1] [xxxxx] [404] [GET] [/wp-login.php] [Blocked access to the login page] [hex:asdsdadaas]
[1734111213] [0.00138] [] [#6134179] [0] [1] [xxxxx] [404] [GET] [/wp-login.php] [Blocked access to the login page] [hex:asdsdadaas]
[1734111223] [0.00170] [] [#7266090] [0] [1] [xxxxx] [404] [GET] [/wp-login.php] [Blocked access to the login page] [hex:asdsdadaas]
[1734111226] [0.00165] [] [#3951801] [0] [1] [xxxxx] [404] [GET] [/wp-login.php] [Blocked access to the login page] [hex:asdsdadaas]
[1734111246] [0.00199] [] [#5247867] [0] [1] [xxxxx] [404] [GET] [/wp-login.php] [Blocked access to the login page] [hex:asdsdadaas]
[1734111251] [0.00166] [] [#8835551] [0] [1] [xxxxx] [404] [GET] [/wp-login.php] [Blocked access to the login page] [hex:asdsdadaas]
[1734111257] [0.00261] [] [#4961932] [0] [1] [xxxxx] [404] [GET] [/wp-login.php] [Blocked access to the login page] [hex:asdsdadaas]
[1734111267] [0.00183] [] [#4787097] [0] [1] [xxxxx] [404] [GET] [/wp-login.php] [Blocked access to the login page] [hex:asdsdadaas]
[1734111280] [0.00181] [] [#2099382] [0] [1] [xxxxx] [404] [GET] [/wp-login.php] [Blocked access to the login page] [hex:asdsdadaas]
Your domain name (3rd field) is missing:
[1734109528] [0.00154] [] [#7984975] [0] [1] [xxxxx] [404] [GET] [/wp-login.php] [Blocked access to the login page] [hex:asdsdadaas]
That means your HTTP server is misconfigured and doesn’t populate the $_SERVER['SERVER_NAME'] environment variable.