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Mod Security Error Upon Login at Multiple Sites [closed]

I am receiving the following error when logging in to multiple websites:

Not Acceptable!
An appropriate representation of the requested resource could not be found on this server. This error was generated by Mod_Security.

I cannot log in to multiple sites and I am a little perplexed; the sites are not in any way related (one is a soccer fan site, another is my hometown tax payment portal service, etc), which leads me to believe both pages’ Mod_Security settings seem to be detecting my IP address as a threat. What on earth would trigger this? I’ve never had this problem until the last few days, during which nothing of note has happened to my computer or my ISP.