This has happened to me a few times recently as well, but I know for sure it wasn't the MC to blame for my ps2's mishap.
I'll share for the sake of anyone that find this happening to them and they also know it's not because of the MC
It happened to me because of my PSX trigger disc.
Apparently, if your trigger disc is scratched, or otherwise damaged, or if for some reason your ps2 isn't reading the disc correctly (in my case it was all three of those things) then the ps2 may not boot the exploit and will instead try to just boot the psx normally.
I assume this has something to do with the ps2 not getting all the information it needs from the disc while reading it, or not being able to get the data in sync from the time it reads the boot file on the mc and the disc ID on the trigger disc, but that's just what I said; an assumption.
For those of you that tried urbigbro's suggestion of cleaning the contacts and reformatting the MC and that didn't work, try checking out your trigger disc for scratches or dirt and if possible try setting up a new trigger disc =)
I'm gonna go clean the contacts for my mem-card now just for the sake of it considering it's probably never been done hehe
Ver. 3 NTSC independence exploited ps2, 120 gig WD HDD, HDLoader .8c, Ulaunchelf 4.04, Blackrhino linux on WD 40 gig HDD