Is the used clock for the Baudrate generator adjustable on an arduino due?
The clock of the transmitter and receiver have to be matched.

In theory, the worst case error tolerable is 1/2 bit per byte.  For the 10 bit byte (1 start, 8 data, 1 stop) that leads to a +/- 5% clock error tolerance that you will find reported.  Note that this actually depends on the specific UART implementation, either chip or internal.  There are various methods of sampling that make some UART's better than others.  It is common to sample in the middle, but some chips have methods to improve noise rejection.

For the receiver side, do not rely on what the terminal program is showing you.  Few applications would manipulate the clock directly; they commonly just call an API with an enumerator for the speed.  You need to know exactly what hardware the receiver side has and its clock.

In practice, the theoretical limit is not going to work.  At high data rates, the noise on the cable will eat away at that safe margin.  More common recommendations will say +/- 1% which allows for noise on the edges of the bit transitions.

The noise on the signal is important at these data rates.  You indicate you are using TXD/RXD which is just logic level.  You cannot connect logic level any significant distance.  If the two are directly next to each other with excellent attention to wiring, you might have a chance.  Adding transceivers will help your chances significantly.

Also, note that changing to 7 bits helps.  Do you need 8 bit data?

So in summary:
- The receiver is more important than the transmitter, but they both have to match.  If you can't find a match of clocks to 1%, you will have data errors.
- At these speeds, the cabling and connection is critical.

This article at Maxim is a good starter Maxim "Determining Clock Accuracy Requirements for UART Communications.