I have a mod that requires Forge for Minecraft 1.5.1, but there's one problem. That is I can't install Forge for Minecraft 1.5.1. The official universal installer jar works by first downloading required files, and then creating a new profile set up with the modded version of Minecraft. Problem is, there's one file it can't download. It tries to download http://files.minecraftforge.net/fmllibs/scala-library.jar but this creates a 500 error (Internal Server Error). So that download is broken. And with that download broken, the universal installer crashes once it tries to copy the downloaded file to the correct folder.
Apparently this is a dependency that is not needed for versions of Forge that are built for newer Minecraft versions, so the webmasters of the website that hosts Forge (and its required files) must have been cleaning up their server, by removing old files, and ended up removing some files required by older versions of Forge. So now (from what I can tell) I can't install this version of Forge. And that means I can't run the mod that I want, that REQUIRES this version of Forge.
Any help in getting around this issue will be MUCH APPRECIATED.
I use MultiMC or Twitch (MultiMC a lot more since Windows 10 issues with Twitch at a time because it didn't trust the Twitch app XD was not happy so I had to find another method if I couldn't access modpacks through Twitch at the time) usually for these types of situations for ease of setting up instances, getting the mods myself like I would for Vanilla and accessing Forge files without any issue. (Not great with 1.2.5 but more so due to the mods I choose probably but is for 1.4.7 or 1.5.2 when I attempted to so I assume 1.5.1 should work. I tested it myself just as I was typing this and it works )
MultiMC feels like the put mods in like Vanilla mixed with the 'get Forge without having to manually get it' to me, while Twitch is more the 'get everything with ease' which is great but not for things like Rift or Fabric which I use a lot now for 1.13 & 1.14 mods unless I count my 'workarounds'. MultiMC just feels like a good middle of what I'm used to for getting mods the old way and putting in, using old versions and so on. While specific modpacks of 1.2.5 I use Technic for.
I do remember I think even with older versions of Forge that it wasn't easy to get them working for a dependency or just remembering how to set it up again with I think the newer Vanilla Launcher and Forge when I wanted 1.4.7 I think it was at the time but didn't come across this issue with installers I think.
Since the Twitch/Windows 10 incident I tried Technic Launcher with premade Forge instances but they aren't great due to what Forge versions come with them and me not updating them due to lack of a way to work out how unless I probably created my own Technic modpack and did that myself.
And I haven't used the older Launcher or tried setting up Forge for older versions from 1.5.2 and older in a while, but when I was in my period of doing so which I still do from time to time, MultiMC is my go to. Twitch is great but hasn't always for me, MultiMC just came to be my Launcher of choice since, but especially due to the Windows 10 app distrust incident. MultiMC is really easy to understand, remove Forge, instances, access configs, worlds, mods, resourcepacks and more. So I just use that, and I use Vanilla for my brief stuff like snapshots to give me reason to still use Vanilla over using MultiMC for everything.
That's just my experience anyways, hopefully you'll find something that works for you. But that's what I've done instead of going to more lengths to get Forge for older versions when using the newer Vanilla Launcher and getting nowhere or more so not knowing where to start/pulling out the old launcher every time I want to use it for older versions.
I found a possible fix for the missing library; in this case the version is 1.6.4 so I'm not sure if it used the same version (2.10.2) as 1.5.1 (it should be mentioned in the crash report, as shown in the first post):
Assuming you have Windows - press Windows key+R at the same time. A small window should appear; type "%appdata%" and press enter. Locate your .minecraft, and open it. Then, open the "libraries" folder.
Enter the folder named "org". If it's not there, create it. Inside that folder, create a new folder named "scala-lang". Open in. Inside that folder, create two new folders "scala-compiler" and "scala-library". Inside each of these two folders you must create a folder called "2.10.2".
Now, download the Scala 2.10.2 zip from here: http://www.scala-lang.org/download/2.10.2.html Under "Other resources" look for the file "scala-2.10.2.zip" - must be .zip - then download that. Open the .zip file, and enter the "lib" directory. Copy the file "scala-compiler.jar" to the 2.10.2 folder within "scala-compiler" folder you created earlier, and rename the file "scala-compiler-2.10.2". Then copy the file "scala-library.jar" to scala-library/2.10.2, and rename it "scala-library-2.10.2".
You should now have added these two files to your .minecraft/libraries folder: