I used to mend modems years ago, we used to get some back from Italy with flash-over damage, it was not a problem with the modems from UK users. . It is quite likely that the flash over damage you see was caused by condensation, and that it has nothing to do with the current problem,as the blue-ish copper sulphate-like deposit would take time to form.
It might indicate that a "transorb" or similar device has failed open circuit.
I'd check all the semiconductors or simply replace them all anyway.
you can get all the bits from Grandata they may even do a "repair kit" if it's a popular enough problem.
formatting link
-they're good on the phone if your not sure what you need
In message , geoff writes