I get an error of Error Loading Entity Tracker: Decoding when I try to view a replay file, and it plays as normal; except there's no blocks, only entities, and the entities appear to be falling through the world. I've tested in singleplayer and been unable to replicate the error. I'm on a modded version 1.7.10, and I hope you can help me!
Please post the contents of the fml-client-latest.log file inside the log folder inside your .minecraft folder to some pastebin site and link to the post here.
Note that this log will only contain information on the last time you started Minecraft, not any previous runs.
The fml-client-latest.log appears to be old, the date being 10/7/2017 at 1:54PM. My last launch time was at 10/8/2017 at 11:48AM, as shown by the latest.log, and the fml-client-latest.log is for a different minecraft version entirely, and completely unrelated to the issue at hand.
https://paste.replaymod.com/fucamodini.scala
There's the content of the latest.log, which I believe is the same as the Game Output, and is currently the only log file that relates to the issue at hand that I can find.
Hm.. yes, that log file seems to contain everything that's usually in the fml-client-latest.log file which presumatly didn't exist back in 1.7.10 (first bug report I'm getting for that version). It also includes both (if they're independent) errors. I don't recall ever testing MP in 1.7.10 which is probably why it's so utterly broken. Thanks for the report, I'll fix it when I find the time to do so (probably next weekend).