My experience is phpBB but typically you have to do the script upgrades first (e.g. SMF and other scripts), and then last test the upgrade of php, MySQL, etc. Do not try to update the server software itself until the SMF is upgraded successfully.
Also, in my experience add-ins or extension that are not part of the forum software tend to not be maintained after years so those also become a problem. I run very few extra "addins".
It looks like Subs-ForumFirewall might be a plugin?
You also need to be very aware of the system requirements for each SMF version release. Typically you need to update the forum scripts first before upgrading PHP because once you upgrade PHP to like 8.x or whatever it's a massive upgrade and not backward compatible. That part doesn't surprise me.
I sent you my email address directly. If the forum is down and you think I could provide some advice feel free to reach out.
Is this the version of ForumFirewall you're using? Last updated in 2017 -
https://custom.simplemachines.org/index.php?mod=2815SOMETIMES, you can get away with disabling these plugins / add-ins FIRST, to allow you to actually run the upgrade of the core SMF. Then review which ones are compatible with the new version of the forum software SMF. Re-enable or re-download the new version if possible.
Exactly which plugins/mods are you using? How many?
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If I was you, I'd look at what all plugins you are running, disable each one ALL OF THEM (on a test board if you have one), and then try the SMF upgrade. See if that works. If it does work, re-enable plugins one by one, update as needed, ALSO disable ones that break the forum or are no longer compatible. If you can get the forum working OK at that point, then and only then consider updating PHP, MySQL, server side scripts.
In a nutshell:
1. Backup your current database and files
2. Disable all forum plugins / mods now, record which ones you were using (and their version numbers)
3. Do a test upgrade to SMF 2.1 (you may have to update multiple times to 2.1.1 then 2.1.2, then 2.1.3 I'm not sure if you can go straight to 2.1.3 from the current 2.0.19 !!!!! ) - you may want to just do one update per day to make sure there are no issues each time rather than doing all at once!
4. If the upgrade works and the forum is working, only turn plugins back on one at a time but update them from the developer. You likely can't put all them back up.
5. If you are good now, only at this point consider PHP / MySQL upgrades, I would strongly recommend you wait a few days after the forum is upgraded to do this to make sure there are no pre-existing software bugs that arise from the other SMF upgrades.