My development sandbox is on Summer '18, so I initially thought this was related to that since this happens on two different machines with IC2 installed. However, I just downloaded the Summer '18 plugin that you made available in another ticket and installed from disk. Unfortunately that didn't help matters.
I first noticed "weirdness" when running All Tests. I would get back error messages that tests were ignored (last count ~102 tests ignored... some variation of: "Test method StoreFrontController_Test.createProgram was never reported as completed. Perhaps it doesn't exist on the server?" (bolded portion replaced with whatever test it failed on)
Switching to Setup on my sandbox in the web browser, in Custom Code > Apex Text Execution, selecting and running all tests appear to complete successfully.
then, problems started happening when trying to retrieve the latest metadata from the sandbox. this presented as a Intellij Modal with the text: "Could not send Message".
So now I'm unable to retrieve any files from Dev.
FWIW, I opened up a ticket with Salesforce, but as Setup is able to complete tests then suggested that I open a ticket with you :)...