I identified the problem: it's because of the special meaning that "0" got for an attachment point (the meaning being: "use default attachment point"), and the fact that OpenSim servers consider "128" (which is the value for the ATTACHMENT_ADD flag used in SL for multiple attachments) just like another attachment point number (so, when the OpenSim Server receives "128", which would mean "Add to default attachment point" in SL, it understands "Replace attachment on point 128").
I already pointed out in the opensource-devel list that the viewers need a way to figure out whether the grid they connect to implements multiple attachments (and/or inventory links) or not: see
this message. Alas, there was no reaction from LL.. It would perhaps be time that OpenSim developers give us, viewer developers, a way to identify OpenSim multiple attachments and inventory links (future) support (if you have any say in OpenSim development, or are in contact with OpenSim developers, please feel free to pass this thought and wish of mine).
In the mean time, I implemented a manual workaround: by default, both multiple attachments and inventory links are disabled in the Cool VL Viewer (v1.25.0.15 onwards) when logged in on an OpenSim grid, unless the corresponding debug variables (OSAllowMultipleAttachments and OSAllowInventoryLinks) are set to TRUE...
Please, try
this binary (untar it in your installation for the Cool VL Viewer v1.25.0.14) and tell me if it solves all the issues for you. Note that the first time you will log in with the new version, you might have attachments rezzing around your avatar but not being marked as attached in the inventory and you will have to "detach" them by right-clicking on them in-world and selecting "Detach" from the pie menu: this is because the viewer (when in OpenSim and OSAllowMultipleAttachments is FALSE) and the server don't agree about the attached status. This status is synced again after the first login (and provided you synced things manually as described above before logging out).