In the printer context menu, there is a log tab where you can enable logging for every print, and there you see the complete communication. It's more or less what you see in the console. It just doesn't get thrown away, and it has every line which is exchanged, so only this is completely to verify what is happening.
The lock part you showed is a bit late. I mean, we see we did send line 3 and then line 4, and after line 3 we also see an OK.
But I see M75 with a file name after it. In an old changelog, I see we did change it back to only M75, so I assume you are not running the latest Repetier-Server version. Is this right? Old Marlin is known to not support M75 with filenames, so this might be already the part that is making problems here.
The last OK in your log is not correct. It should only have OK or OK, but with an !< Something is more likely to have gone wrong here. But first I would try to update the server to the latest version so M75 is only M3 which should be supported by your firmware.
