Can an improperly attached ledger board be flashed correctly without removing the existing ledger?
@trebor nirom , sorry I did not see your comment was going through old questions and found it.  One product I have used is “cop-r-shield” it is thin copper some what easy to form but more expensive than the “tight seal” aluminum flashing tape that I regularly have used. The aluminum is thin so if poked with a stick it can be punctured I pressed it tight up under the siding and down the wall then over the ledger. It lasts and did the job on that deck , no more water getting in the walls. The copper cost ~40$ for a roll and the aluminum is ~10$ other than one of my kids poking holes in a section it has held up for close to 20 years. Where the holes were poked through I added another layer about 6” wider on each side, it will stick to itself.