Hi angfal , Welcome to pinside!!

The most likely cause is one of these switches on the Game Over relay or the Tilt relay

For a switch to work 3 things are necessary:
1) When open, there should be a small space between the contact points (duh!)
2) When closing, the long blade's contact point should push the short blade's contact point enough to move the short blade
3) The contact points should be clean, which they usually will be if #2 is happening

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I had the same issue with my machine. I found that the Jones plugs (under the playfield) were dirty. I cleaned them with a stainless steel wire brush and it fixed the issue.