I understand the frustration.
The problem with a scenario like this is: Most time those who experience the issue are happy once it works for them. Then there is not enough info to fix the origin. Sometimes it is also a factor outside the IDE (in which case ideally a better error message would be implemented, but that also depends on enough feedback).
To add upfront: The development team has different people for different parts, different OS, .... I am likely not the one who maintains whatever code is involved. But if I get enough info, to pin point it, I help adding it to the bugtracker, so an improvement can be worked on.
Unfortunately with the size of the project it can prove a lot of work to get the info.
Additionally: stretch-backports is not maintained by the Lazarus team. No one from the team knows how they build the package. Though I doubt that this is the cause of the problem.
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From the log it looks like you run into a 2nd error (different from the original issue).
It looks like it did compile the IDE. There should be a file "lazarus" in /home/slyde/.lazarus/ (or /home/slyde/.lazarus/bin - I am not perfectly sure, I occasionally use linux, but my main machine is windows).
If you use "startlazarus" it should start the new IDE. If you start "lazarus" this will not work. [1]
At least the log says
Hint: (lazarus) TMainIDE.DoRestart
And that only happens either after a successful rebuild, or if you select it from the menu. Since there is no error regarding the build, I assume the former.
Is this correct, or did you trigger the restart in any other way?
This means we don't know why it did not find the file before.
But you said you did try lazarus before. So it is possible an old file was there. And such old files can apparently sometimes cause the "not found" error.
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The crash happens while the IDE tries to close down.
Unfortunately the log doesnt have enough info on what exactly went wrong.
If you can build your IDE with -gl in the options ("Configure Build Lazarus"), then next time it happens the log may have more info.
Also how did you start the rebuild? From "Configure Build Lazarus", or from "add packages" or ...?
Did you do any other stuff in the IDE, while the IDE was rebuilding, more to the point at the time it crashed?
The only thing that your log indicates is: At the time of the restart, the IDE was reacting to a menu-item or toolbar-button having been clicked.
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[1]
The reason you need startlazarus is that the install is done as root, and not writeable to your user.
SO the original lazarus can not be overwritten by you. The replacement goes into your user folder. startlazarus will find it there.