Illustrator CC 2018 (22.0.1) preview issues

Hello, we are having trouble previewing in Illustrator.

Facts:

Mac OSX El Capitan 10.11.3

Imac 21,5", end 2013, 2,7 GHz i5

8Gb RAM

Intel Iris Pro 1536MB

Classroom environment

running latest version of Illustrator since 1,5 week.

I switch off GPU performance, due to the fact that scanned objects do not fade in template mode (redrawing originals)

problem:

even with small, basic files Illustrator switches to Outline mode and can not be switched back to preview mode... like it has used all of its caches.

quiting illustrator, logging out and recommencing is the only, not so productive, solution.

what could be the reason and solution for this problem.

thx

Ann