Mizzion

it's possible that with the new updates to the download page, the author may have decided to move away from Nexus, I fear a few Mod authors may do so now. If that's not the case, It could just be broken, and they don't have time to fix it right now.

Many mods have already been updated to work with the new update. You can check https://mods.smapi.io to see if the SMAPI mod you're asking about has been updated or if it's broken, it may have an unofficial update (Depending on if it's open sourced or not).

I have released an unofficial update to the currently broken mod. You can find it on the official forums in the following topic https://community.playstarbound.com/threads/updating-mods-for-stardew-valley-1-3.142524/page-6#post-3277908

You can always make a copy of your 1.2 folder and place it somewhere. Then update to beta with the steam/GoG version. Then when you want to play 1.2 Open the SMAPI in the copied 1.2 folder.

Are you playing on the new 1.3Beta? If so, those mods haven't been updated yet. You can see which mods are compatible by visiting the following link. https://stardewvalleywiki.com/Modding:SMAPI_compatibility#For_players_.28Stardew_Valley_1.3_beta.29