First of all ,I have many years of experience in both Maya and Arnold and my workflow until this moment was Maya 2015 with the latest compatible mtoa...
But about every three years I update to a more recent version of Maya and test for a little while before switching.
The issue/bug I 'm having is that I can't render aovs in an exr format. It's a very strange bug that happens always in every scene.
Freshly installed Maya 2018.5 with resetted preferences and many versions of Mtoa. It only happens on a Windows 8.1 pc. Specifically it writes the exr but in an unreadable file(Nuke or PS). If I change the driver in any other file(tif,jpg,etc) it writes normally. I did the tests with most of the available aovs. The Z aov works,that has data type float(i don't know if it's changed from the previous versions but the pass was written on the rgb than the alpha). I tried every parameter tiled, different compression, merged AOVs....
Beauty pass is fine, Arnold render view AOVs is fine,
To narrow down the problem and test, I uninstalled/install different versions of Mtoa (3.0.1 to 3.1.2.1). Everything is ok in a Windows 10 pc. In the same os there are Maya 2013/2015/2016/2018. The first test I would do if I could is to test in another win 8.1 machine, but I only have access to win 7 and 10 setups.
So the bug happens in Win 8.1 . Maya 2016/2018 and Mtoa 3.0.1 to 3.1.2.1