Bosch Dishwasher AquaStar 6.0L - Error E21 and E24

I have been having problems with dishwasher for a month or so now.

The machine is plumbed in close to the sink. The drainage hose is connected to a spigot on the side of the sink u-bend.

We schedule our dishwasher to operate overnight. When I come down in the morning I will see that there is error code E.21 and when I open the door, the contents have not been washed and the tablet is still in the locked compartment.

I have tried a number of things including...

Cleaning the filter

Removed the white plastic impeller cover and checked that the impeller is turning freely (it moves freely.)

In an attempt to rule out blocked or kinked waste pipe, I've pulled machine out from under the worktop and disconnected waste pipe from the u-bend and pulled pipe out so that I now have machine in middle of kitchen and waste pip going directly into a bucket.

Ive used a pump to blow/suck the waste pipe and drain out the water.

Once all visible sump water is removed, I set the program to quick rinse. As the sump is empty, no additional water is pumped out during the initial phase of this program. The machine fills with water and 15 mins later I can see water clearly pumping into the bucket. I took from this that a) the pump is working and b) the waste pipe is not blocked. I repeated this test twice (varying the height of the highest point of the waste pipe and all seemed ok.)

My next test was to run a wash program (intensive vario speed). I did not sit and watch for the whole cycle, but the water initially drained into the bucket and then continued onto the wash. When I came back at the end of the cycle, there was error code E.21. The bucket contained waste water and the dishes were clean and dry.

There was no more water in the sump than I would have expected.

I ran intensive vario speed program again. Once again the program ran through and ended with a E.21 error code.

It seemed as though the machine was washing and draining but ending with an error code E.21

That evening I loaded the dishwasher and scheduled for overnight run (dishwasher still in middle of kitchen and draining into a bucket,. When I woke in the morning the dishes were dirty, the tablet had not dispensed and there was an error code E.24.

Ive exhausted all the things that I am aware of and from my tests it seems that

a) the pump is working

b) the waste pipe is not blocked

c) the impeller is turning ok

d) the issue is no caused by my waste plumbing (as Im getting error codes with waste pipe into bucket)

Any other advice on things that I can try?

Any advice from experts on what/where the problem lies?