Glad to hear you have found the culprit, now to fix it could be tricky.
If you have a set of nut slot files, you can re-file the slot so the bridge end is the high point. This is normally done in two parts, a shallow angle for the front of the nut and a steeper angle at the back to help guide toward the posts.
If you cant correct this without lowering the string height too far, you will probably need to replace the nut and start again with the filing. This means carefully removing the nut, cleaning out the slot and shaping/installing a new nut.
I am hoping it can be fixed without having to go this far. :-)
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