Thanks everyone. After a period of testing, we were able to replicate the problem on our end. We subsequently went looking for any vulnerabilities of the site that could have opened up for this kind of exploits.
I'd like to stress that iModeler is an ad-free service at this point. Although we might change it in the future, we would most certainly not introduce ads without telling it to the users.
I have asked the knowledgeable people from our tech support to take a deeper look, and they eventually reported that they were able to locate the source of the problem, patched it and secured the offending server component.
The impact of the exploit seems to have been moderate and limited to ocassional redirecting of user clicks to ad sites. It is at this point unclear how many users were affected (apparently several).
Please accept my apologies for all the inconvenience. We are taking the security of this site very seriously, and will follow-up with a thorough expert review for any other problems.
I'm also sincerely grateful for bringing the issue to our attention.
Please remain vigilant and let me know if there are any remaining signs of trouble. Your cooperation and understanding is very appreciated.
/Martin