We made a pretty big architectural change in r42 (the original release notes are pasted below in case you missed it), and unsurprisingly we ran in to a couple of issues with it. We're doing another minor update today to fix two more issues that were found. We apologise for any inconvenience, but this is a necessary change to ensure C3 is robust in future. Hopefully we are close to solving the last few issues of this change!
We're keen to get all architectural details sorted as soon as possible, to make sure we have the most reliable base possible for the future. As part of this, we've done a significant reworking of preview mode to get it to run on preview.construct.net instead of editor.construct.net. This is to ensure that games are sandboxed from the editor as much as possible, helping improve the security, reliability and performance of previewing games.
It might seem like a minor change - it's just a different URL in the address bar! - but this is actually involves a lot of refactoring. Since preview is on a different origin (it's on a different domain to the editor now), browsers impose strict limitations on how the two windows can communicate and interoperate. This means we've had to adjust a lot of the internal details of preview mode to ensure it works within these strict limitations.
Since this involves significant changes, this release is mostly just to test the new preview mode (although we threw in another two bug fixes). We've done our best to test it carefully, but there may be unanticipated issues with the change. So please give it a spin and let us know if you run in to any issues!