IE new user subscription problem

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I've received several complaints from new users who try to subscribe on my site.
It seems that the problem is related to Internet Explorer.
If they use this browser and try to subscribe and they see the message:

Please enter the six letters or digits that appear in the image opposite.

There's nothing to see. No digits nor letters appear in the image.
Users who browse with Mozilla as I do for example, don't have this particular problem.
So, is there someone who regocnizes this problem and is able to help me?
In the meantime, I recommend the new users to use another browser or to send me an email so I can create their userid and password. Off course, I guess a lot of potential users don't subscribe at all due to this problem.

Thanks in advance for solving this problem!