When I first load into the game with Mod Mash, I get this error in the log, and in the startup screen, after which I must disable the mod. I do have other mods installed, so not sure what might be conflicting if so.

28.816 Error ModManager.cpp:1332: Failed to load mod "modmash": modmash/data-final-fixes.lua:29: attempt to compare number with nil
stack traceback:
modmash/data-final-fixes.lua:29: in function 'local_create_entity_loot'
modmash/data-final-fixes.lua:40: in main chunk
28.819 Loading mod core 0.0.0 (data.lua)
28.988 Checksum for core: 1801404058
29.078 Error ModManager.cpp:1332: Error in assignID, recipe-category with name 'crafting' does not exist.

Source: default (utility-sprites).
29.209 Initial atlas bitmap size is 16384
29.209 Created atlas bitmap 2048x432 [icon, not-compressed, mipmap, linear-minification, linear-magnification, linear-mip-level]
29.216 Texture processor created (2048). GPU accelerated compression Supported: yes, Enabled: yes/yes. Test passed. YCoCgDXT PSNR: 35.83, BC3 PSNR: 33.82
29.232 Parallel Sprite Loader initialized (threads: 11)
29.287 Sprites loaded
29.301 Generated mipmaps (5) for atlas [0] of size 2048x432
29.301 Custom mipmaps uploaded.
29.306 Factorio initialised
29.306 Discord overlay detected (DiscordHook64.dll)
29.307 Mods to disable:Failed to load mods: modmash/data-final-fixes.lua:29: attempt to compare number with nil
stack traceback:
modmash/data-final-fixes.lua:29: in function 'local_create_entity_loot'
modmash/data-final-fixes.lua:40: in main chunk

Mods to be disabled:
• modmash

I am playing in 0.17, if it matters.

It does. Im not sure what version of mod mod you are using but this does sound familiar and fixed in 18. It is mod adding items and doing somthing odd mm wasn't prepared for. So long ago i dont even remmember what it was. Which version are you using i may be able to figure it out and let you know how to fix but wont be publishing a 0.17 release

I'm in version 0.17.79

Do you mean 0.17.97? if not try that having pc issues

In the betas tab on steam, 0.17.79 is the last 0.17 version I can see.

Odd, it is in the download section. I cannot actually help at the moment my pc has decided to go and die on me

That's alright. I decided to just move up to the latest 0.18 version anyway, so here's to hoping all goes fine installing it in 0.18