Is there another PC in your network (apart from the one running the wamp server) where you could try the test:
http://192.168.0.121/arduino/getdata.php?sensor1=1&sensor2=0&sensor3=1
It just could be an access rights issue which prevents the arduino code working although, from the response you got back, it looks more like a malformed HTTP GET problem.
As for the alternate fail/success issue
- comment out the two client.stop() statements in your latest code.
- add a new client.stop() statement as the first statement at the beginning of the loop()
To see if that clears the issue.
The following is a better template to use where you are making repeated http get calls:
https://www.arduino.cc/en/Tutorial/WebClientRepeating . Maybe use the structure there.
You've said you can see the web server log files. What do you see for the last time you ran your script ? Do you see the full URL your script sent ? If so, post it ?