On re-reading my earlier post about the CAPTCHA mod, I realize that it needs some additional clarification.
Firstly, I'm a big fan of this mod and its author (funkey). The mod is easily installed and does exactly what it says it will. And the author has taken the trouble to update it for each new release of Zen Cart.
However, through no fault of the author, this is a mod that is currently very sensitive to being matched to the correct version of Zen Cart since the code upon which it is built has gone through some large changes. So if you use an earlier version of the mod on a current version of Zen Cart (and especially if you use the 099.135 version on a 1.3.6 or later Zen Cart) you will lose the benefit of corrections made to the underlying Zen Cart code and reflected in the mod. Using a newer version of the mod on an older Zen Cart might also have adverse results that are less easy to predict.
Given this, anybody having the types of problem described above in a 1.3.6 or later Zen Cart with the CAPTCHA mod installed, should first try to resolve it by installing (or reinstalling) the version of the mod that matches their version of Zen Cart.
If you're running version 1.3.5 or earlier, then you will need to upgrade Zen Cart to get the solution introduced in 1.3.6. And of course when upgrading your Zen Cart you will also need to upgrade your version of the CAPTCHA mod.
A bit complicated I know, but if it's not clear, please post back here.
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(Questions answered in the forum only - so that any forum member can benefit - not by personal message)