This bug came up in two different behaviours, but I think, they have the same base:

First way:
1. Select an account
2. Start new transaction with Strg+T
3. Enter some value and a target account
4. Submit transaction

--> One account is lowered/raised on the entered value, but in the other account, an transaction with no value is added (sometimes it works)

Second way:
1. Open an account with double click
2. Enter a new transaction in the empty row at the end
3. You can not leave the row and right clicking --> Buchung Speichern (guess Save transaction in english version) does not do anything

From my point of view, it has something todo with currencies of the accounts. As gnucash shows an info on opening the accounts, that it can not determine the currency and uses the system default.

I do not know the reasons, causing this error, because I use the file since 2006 without any problems.

A resolution is very appreciated!