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Make sure you don't have any browser plugins or other software installed that may interfere with Issuu. This includes ad/Flash blockers, adult content or similar censoring filters etc. Note that these kinds of software may also be controlled by a system administrator if you are on an office network.
Generally we work perfectly on most popular open source setups. However, if you are on a Linux or open source system you may experience problems. Our best advice is to try searching online for help.