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My cli wallet.exe opens for a few seconds and closes automatically without giving any error message.
There is no problem with mining so far. I followed the tutorial by @tuck-fheman for windows.

Edit : I ran cli_wallet.exe from commandline and found out the error I receive is as same as the OP's error and adding your value to the rpc-endpoint didn't help. Btw, The miner is already running. Should I stop the miner for the change in configuration to take effect for the wallet?

Yes, you may have to restart after editing the file. Try putting a space in front of the command too, that is how it is in my config file.

Ok, I'll give it a shot.

Edit : It is running now but I get the following message asking to use the set_password method to initialize a new wallet before continuing.

Logging RPC to file: logs\rpc\rpc.log
Starting a new wallet
780378ms th_a       main.cpp:154                  main                 ] wdata.w
s_server: ws://localhost:8090
781384ms th_a       main.cpp:159                  main                 ] wdata.w
s_user:  wdata.ws_password:
781407ms th_a       websocket_api.cpp:88          on_message           ] message
: {"id":1,"result":true}
781414ms th_a       websocket_api.cpp:88          on_message           ] message
: {"id":2,"result":0}
781421ms th_a       websocket_api.cpp:88          on_message           ] message
: {"id":3,"result":2}
Please use the set_password method to initialize a new wallet before continuing
new >>> 781424ms th_a       thread.cpp:95                 thread               ]
 name:cin tid:400

some else now about this? "please use the set_password method to initialize a new walet before continuing"... what command or what can write there?, in the cli_wallet.. i guess that its the final step to start mining.. pls answer this, i gonna pay you! xD

I think you need to enter the following command first
set_password password(or your desired password)
then, use that password to login to wallet