No debt is allowed. At all.
Ok but they do they allow any potential debt to be avoided from private investment rather than purely via natural income? Going by Wolfburg's financial backing from Volkswagen and Leipzig with Red Bull, it appears they do?
I'm not sure. One party/company cannot own more than 49% of a club, though, with exceptions only made for Wolfsburg and Leverkusen because they were originally set up as teams for companies.
There's a rule that if you run into temporary debt (eg from buying a player) you cannot buy another player until you sell a player/players to service the debt, which suggests that, no, an owner or investor can't plug the gap, but I can't find anything that makes it 100% clear