When trying to install Agent 5.5.1 on a machine where 4.8.0 is installed the task is successful but the machine is out of control afterwards. However when I try to install 4.8.0 again the problem is solved immediately after seconds.
Regarding MA 4.8, when it was supported the minimum required was 4.8 Patch3, is this the version you have? (If not can you updgrade to P3 and upgrade again?)
Assumming you are working with v4.8 p3, the logs from the 'new' v5 install would hold details about what is happening with the new agent. Can you review the following logs see if they report error messages, share with us so we can have a look to help?
Logs to review (no particulare order):
I suspect and so wonder if you have messages like the examples below:
Error: UV connect Failed - uv error <34> <no such file or directory>