There is no well defined financial distinction between insolvency and whatever you want to call the situation in which the marked to market net value of a bank's assets goes negative due to a fall in prices during a liquidity crisis.† There almost certainly were such situations during the last crisis, when a bank's book value went negative, and yet it was not deemed to be insolvent.

That's because in practice insolvency is not so much a financial concept but an accounting and legal one, and in those domains it refers only to situations in which a corporation cannot meet its financial obligations as they come due. In ordinary circumstances that convention gives corporations some leeway to re-negotiate their obligations to stave off insolvency. But during a liquidity crisis it means that an institution whose book value goes negative (temporarily?—who knows?) will or won't become insolvent in part depending on whether third parties are willing to lend to it to plug the hole that exists in its books at the moment. That means that solvency during such periods is a bit of an artificial thing, depending in part on the of vagaries of the marketplace, as well as the judgement and munificence (or whim, if you'd like) of central bankers and other governmental actors.

> Without fractional reserve banking, there cannot be credit.

This is not true. There can be credit, just not with the simultaneous fiction that creditors retain access to the money they've lent. Bond markets and old-fashioned money-market bank accounts operate without that fiction, for instance.

† — Or for that matter during a classic bank run, which is another form of liquidity crisis. A typical bank operating on fractional reserve is solvent in the sense that over some indefinite future time horizon it should be able to give its depositors their money if they demand it, since the money the bank is owned in loans exceeds the money the bank owes its depositors. But a bank does not enough money in its vaults to pay all its depositors if they all want it back at the same moment; if no third party is willing to lend cash to the bank suffering the run ("provide it liquidity"), then it will become insolvent, no matter what its book value.