Battle Test opens Title Screen, even when plugins are off

I've been having this problem for weeks (apparently there was an update?). Every time I try to test a battle, it takes me to the title screen instead. I've tried turning off each plugin individually, and turned them all off at the same time, but nothing works.

If I create a new project, though, it works fine. But the project I've put about 80 hours into so far doesn't have a battle test anymore.

If there's no way to fix it, it's not the *total* end of the world, but it would be a massive inconvenience if I can only test my battle by creating an event with it and changing the player's starting position. Is there any way to fix it?

And I'm using MV via Steam, so I don't know if rolling it back to a previous version is possible.

(Plugin list below, the Sylven one is a similar one to Yanfly's plugin that allows players to change armor in battle)

That usually means you didn't update your project. You can read how to update your project here.
Alternatively, you can rollback to a previous version. Right-click on MV in your Steam library, click on Properties, go to Betas and choose what version you want to downgrade to.

So updating my game didn't work (I had opened it in 1.6.1 before I knew about the update, so I dunno if that affected it), but downgrading did! I didn't know you could do that on Steam, thank you!

in that case you made an error on updating the project.
One question: did you copy the rpg_core.js file when updating?
e.x. in the description stands for "example" but some people wrongly read that as "excluding"

I did, yes.