Using Forge 2309, CC 581 and UtilityWorlds 1.1.0. The worlds themselves (mining, void and garden) all work fine, generating no terrain in void and a superflat in the other two. The portals work fine when first traveling to the worlds. When using the return portal, however, I went back exactly where I was, except no terrain was generated, I couldn't move or fly anywhere, and F3 said I was "Outside of world". I had to /tp out of range of rendering in order to start generating chunks again.

The minimap (Xaero's) and its waypoints are unusable when "out of world", but work again the moment I /tp successfully. Without the minimap installed I get the same portal results.

Latest.log (no crash) - https://pastebin.com/WvDX8RJ1

What the portal blocks normally look like:

What it looks like when I get back:

How many times do I have to say that latest.log is not useful for me?

latest.log contains only INFO and higher log level. I need DEBUG/TRACE log level, which is in fml-client-latest.log

This is apparently something very wrong when going between dimensions.

fml-client-latest.log now has something else, but here's fml-client-1.log which ought to have at least some of what you want. (2 apparently does not.)
http://depositfiles.com/files/7245awgmj

Now that is some shady website... next time, coud you use something like dropbox?

Should be fixed in f76103f. Comment here if it's not.