malware warning from Avast antivirus

[REDACTED]

When I go to the o2.ie website (my mobile phone provider) to send webtexts I get a malware warning.  I've been with Avast for almost a year now and haven't had this problem until a few weeks ago. I contacted o2 but they say their website isn't compromised so they suggested changing browser, which I did but the warning is still there. They then suggested it must be the antivirus software. Can you help please?

Colin

[Pondus]

what is the message from avast?......you may attach a screenshot


Sucuri  http://sitecheck.sucuri.net/results/www.o2online.ie/o2/
Warning entry: WARNING:500ERROR   http://labs.sucuri.net/db/malware/500-error?v1

Description: This error happens when your site generates a 500/501/503 internal server error when we try to scan it. It might be a temporary error, but it can also be related to a malicious injection gone wrong and breaking the site.