Someone was trying to hack your site. The address they used ended in .gif before the question mark. You have also set "Show errors using a customisable HTML template" to Yes. In this combination of events the attack is being blocked but due to the way Joomla! and the customizable HTML template works you get an error.
Set "Show errors using a customisable HTML template" to No.
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
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