I got very strange access logs on my apache, like

[17/Aug/2021:07:34:24 +0000] "GET http://51.83.140.237:9933 HTTP/1.1" 404 0 "-" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; tr) Opera 8.50"
[17/Aug/2021:07:34:23 +0000] "GET http://51.83.140.237:9933 HTTP/1.1" 404 0 "-" "Mozilla/5.0 (Windows NT 6.3; WOW64; Trident/7.0; rv:11.0) like Gecko

For tests, I disabled all web pages (removed folders) and still got all the times this access. After a while apache stop working (without crashing, just stop responding).

Any ideas where I should check ? What can I do? What the F* is that ?

Thanks in advance for your help Regards