I use open source, and my version of Firefox has given warnings about the site on log-in for quite a long time (several months) - if you're interested in what they say, this is the address:
https://support.mozilla.org/en-US/kb/insecure-password-warning-firefox?as=u&utm_source=inproduct
As I see it (only personal advice, given without any guarantee of completeness or accuracy), the problem appears to arise from the site not using a more recent secure internet web address protocol (with a web address beginning "https"), but still using the older, less secure "http". This could mean that if the site is poorly scripted, or if it links to advertising sites which are accidentally or deliberately scripted in a way that allowed people to harvest log-in information, eavesdroppers could pick up users' passwords or web addresses.
Theoretically, this could allow other people, or web robots, to post bogus messages in the name of genuine users - but that is more likely to be an issue on mass-use social media sites, than on a fan site with only a few score regular posters.
(Again, speaking personally) I would only see it as a potential problem for most people, if they:
A Use the site to post credit card information (unlikely to apply, here)
B Use the same password here that they use on other, more sensitive, sites (could apply - but not recommended practice in any case)
C Use a computer which has no effective protection against malware (getting rarer these days, as the need for protection has been widely publicized, and often sold with new computers)
If any of those apply, it would be wise to change the practice - if it doesn't apply, it's pretty likely you could go on using the site safely (but at your own risk, of course) - as I have done