Restoring a session only shows blank tabs, same with opening last closed tab

Question

I opened a link from Trillian in the middle of loading a session with the Session Manager add-on, and now none of my sessions will show anything when I start Firefox or when I reload a session using Session Manager. The number of tabs that the session loads is correct, just none of them have anything except New Tab and nothing in it.
Additionally, the closed tabs list doesn't update, or when I load the session using Session Manager, trying to open a closed tab will open another blank tab and turn that entry in the closed tabs list into a blank tab.
Error console shows this when I open any page:
> Frame Script init
> "Cu" is read-only
And this when I close any page:
> Warning: attempting to write ##### bytes to preference recentlyClosedTabs. This is bad for general performance and memory usage. Such an amount of data should rather be written to an external file.
I've tested this with and without Session Manager enabled, and it's the same behavior for both session restores (correct number of blank tabs). I've tested in safe mode and sessions appear to work normally (I can see closed tabs and restore them).
Also, Session Manager saves the correct titles and URLs of the tabs when it saves to its external storage, even though they're "blank", so I suspect the issue is something preventing the writing of an important file in my Firefox profile, but I'm not sure what or how to fix it.
Lastly, Session Manager's add-on page says it might help to record the error console while disabling/enabling the add-on, so here is that log:
Disabling:
Duplicate resource declaration for 'specialpowers' ignored. chrome.manifest:32
Duplicate resource declaration for 'gre-resources' ignored. chrome.manifest:34
Duplicate resource declaration for 'services-sync' ignored. components.manifest:168
Duplicate resource declaration for 'services-common' ignored. components.manifest:170
Duplicate resource declaration for 'services-crypto' ignored. components.manifest:171
Could not read chrome manifest 'file:///C:/Program%20Files%20(x86)/Mozilla%20Firefox/chrome.manifest'.
Duplicate resource declaration for 'pdf.js' ignored. pdfjs.manifest:1
Duplicate resource declaration for 'cookieswap_res' ignored. chrome.manifest:3
Duplicate resource declaration for 'cookiesmanagerplus' ignored. chrome.manifest:3
Duplicate resource declaration for 'personas' ignored. chrome.manifest:7
Duplicate resource declaration for 'abduction' ignored. chrome.manifest:2
Ignoring obsolete chrome registration modifier 'xpcnativewrappers=no'. chrome.manifest:16
Duplicate resource declaration for 'dta' ignored. chrome.manifest:2
Duplicate resource declaration for 'dwhelper' ignored. chrome.manifest:75
Duplicate resource declaration for 'tabmixplus' ignored. chrome.manifest:3
Duplicate resource declaration for 'sqlitemanager' ignored. chrome.manifest:7
Duplicate resource declaration for 'greasemonkey' ignored. chrome.manifest:15
Could not read chrome manifest 'file:///C:/Program%20Files%20(x86)/Mozilla%20Firefox/browser/extensions/%7B972ce4c6-7e08-4474-a285-3208198ce6fd%7D/chrome.manifest'.
1417333357797 Services.Metrics.Provider.org.mozilla.addons WARN Add-on type without field: userstyle
1417333357798 Services.Metrics.Provider.org.mozilla.addons WARN Add-on type without field: greasemonkey-user-script
Enabling:
1417333394803 Services.Metrics.Provider.org.mozilla.addons WARN Add-on type without field: userstyle
1417333394804 Services.Metrics.Provider.org.mozilla.addons WARN Add-on type without field: greasemonkey-user-script

Answer 1

OK I managed to fix this myself so I'll post what I did here:
The problem was caused by TabMixPlus which also has its own session manager, so I had to reinstall that addon.
I also had to disable/re-enable every addon and delete all my cookies/cache because of some other strange issues that were happening, but whoever is reading this might not need to do that if they don't have other problems.