So now I'm digging for solution.

After creating a matrix that works and executing, so the A matrix works, I run lprint and it returns this:
Matrix(3,3,{(1, 2) = 1, (2, 3) = 1, (3, 1) = -1, (3, 2) = -4, (3, 3) = -6},
datatype = anything,storage = rectangular,order = Fortran_order,shape = [])

Copying into notepad returns nothing unusual
A := Matrix(3, 3, [[0, 1, 0], [0, 0, 1], [-1, -4, -6]])

However, as stated before, starting from scratch and executing we get the error.  Running lprint returns a simple one worded answer
        

However when you create another matrix from the context panel which fails produces an rtable lprint result

*****edit added*******

Pasting into notepad from scratch produces what Preben Alsholm is showing

A:= [ [[0,1,0],[0,0,1],[-1,-4,-6]] ]:

copying that into maple and executing produces an empty number error.

.. and will cause any further matrix constructions to fail

A list of a listlist is allowed.  I tried inserting spaces every which way possible and could not replicate the error.  I copied and pasted as well, however it didn't paste a list of listlists it pasted the matrix as it appeared.  

If PainedMushroom could describe how he entered the Matrix or list of listlists, it would help in understanding how the error came up. 

Strange, if I load your worksheet right away and enter on the Matrix I get the error, and then on any other subsequent use of entering a Matrix I continue to get the error - even if I open a new sheet.

Now if I restart Maple then enter a sample matrix - ok no problems, then open your sheet in the same session and enter on the matrix it works fine. 

******edit added****

Within your worksheet, if we run a new matrix and execute, then execute your matrix it works fine.  Only initially executing your matrix will the error present itself after which matrix will not work until a restart.

@PainedMushroom Restart Maple and try again.  I uploaded your worksheet saw the same issue, then tried to enter a new matrix and got the same issues.  I restarted Maple and then I could enter a Matrix with no problem. 

I've now again tried loading your worksheet and I cannot reproduce your error.  Restart will likely fix your problem.

@Carl Love ah, thanks. 

Curiously how are you getting Maple to convert from MPa to psi without any conversion command and vice versa?

Ah, ok so this goes back, way back but the answer was exp(Pi)-Pi

https://mapleprimes.com/posts/41923-Another-Math-Comic

In the GUI, it would be nice if you there was a quick shortcut to enter equations you've entered previously

perhaps pressing CTRL and the up arrow could cycle through all your previous entries? 

@Carl Love Even I forgot about killing the kernel.  I think he may have found your answer to work for him and he's just forgotten to respond, he is a super long time user so he should be back. 

To clarify for the op (he's probably using windows) under the task manager he needs to end the mserver.exe processes.

Currently there is no option for drawing a non free hand partial circle, adding arc option or partial erase would work.  Currently the eraser tool erases the complete object.

Resizing the image also scales the drawing - it would be nice to have the option to change the frame size of the canvas while leaving the drawing scaled as drawn.

After leaving Maple 2021 open for a while without doing anything.  All of a sudden it's fast.

So it appears now maybe there's some processes going on causing Maple 2021 to be slow when it's initially started.  More looking into it tonight.

@Acer - fair enough

So this seems to be a Maple 2021 issue?  I will have to roll back to 2020, see if the issue disappears.  Does this happen with Maple 2022?

@tomleslie covid fog I guess.

@Anthrazit Sometimes I've found the highest resolution setting does not offer the best quality, sometimes stepping down one resolution setting gives better results.  This could be the reason why (as you mentioned above for a vertical screen) the resolution appears better.  Just a thought. 

I'm using a 15" laptop set at 1200 x 800 and Maple 2020 text appears good.

An ancient Toshiba Tecra S10 running Windows 7

@Anthrazit Check windows performance settings and check the smooth edges of screen fonts.

Try also turning on cleartype in the control panel.  type in cleartype from the windows start button and make sure it's checked.