ModuleNotFoundError: No module named 'sage'

edit

last spring I compiled sagemath on 2 machines, 1 with debian testing, 1 debian Sid. Both worked. Now none of them works anymore, even if trying to launch exactly the same way (from xterm):

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Traceback (most recent call last):
  File "/home/lambada/MonSAGE__10_3/sage/src/bin/sage-ipython", line 9, in <module>
    from sage.misc.banner import banner
ModuleNotFoundError: No module named 'sage'
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Something modified by updates-upgrades??