<p>I was able to connect my terminal program (Putty) to my 3D Printer (Creality Ender 3 Pro) and was able to send G-code commands to my printer and it obeys.</p>
<p>Now suppose I sent long command like</p>
<pre><code>G29 ; auto bed leveling
</code></pre>
<p>and it is still executing. Printer writes me</p>
<pre><code>echo:busy: processing
</code></pre>
<p>How to interrupt it?</p>
<p>I tried to send <code>M0</code>, but it didn't work.</p>

Photo interpretation: I understand that the image on the right presents the actual error: it happens on many layers, following the red "Starting Point" line (so above it on the photo). Interesting issue. Btw. which side of the print is the bottom (zero layer)? - I believe the left side, and the right side is the top of the print.
The problem seems to apeear only in the middle of print. Top and bottom layers are usually printed slower, this could be some hint to find the reason. Also, I guess that the vertical cross section is not just a rectangle, but is wider on the top part? - then print parameters (e.g. speed) may change because of overhangs.
Could you share what type of filament do you print with? For example flexible materials will specifically react to the pressure and should not be forcefully retracted and pushed.
What type
Is there any exccessive material anywhere on the table? Like oozing or stringing? Do you see any material lost before printing the first layer? If not, the retraction is good enough or even too heavy - then try to minimize it: you may go down until you see any oozing, and then check the wall.
Could you share what range of parameters you have tried- especially the speed and retraction values? Jerk and accelleration? Do you use coasting or other pressure affecting techniques?
What is the hotend diameter? - the specs says 0.4 mm and the direct extruder. And (in comparision) what is the wall's line width for extrusion? What is the real width of this wall? (Is it properly sliced and reflected in G-Code? I advice to use G-Code viewer and inspect the details in given area.) What is the infill %, and how many wall lines do you have configured? And the layer height?
I hope this is of some help for diagnosis.