I looked up plantation soil and if it's the kind I found it is coco fibre as we suspected. If it's the problem then I wouldn't replace with that. You mentioned earlier there was another substrate underneath, do you know what that is?
The plantation soil I found is marketed for reptiles etc. but says you can use it for plants. I couldn't find anything about fertilizer etc. in perhaps because it's considered pet substrate and doesn't have any additives for plants.
Are any of the plants showing brown leaf tips or edges? If they're not and still seem to be OK I would prepare some of the coco in the same way as before, put it in a pot and enclose it in a clear plastic bag. Watch it for a few days and see if anything develops on the surface. If you're still misting or watering the terrarium then do the pot at the same time.
The bag of coco fibre should have a contact phone number on it, if it's toll free from where you are then I'd give them a call. Coco fibre/coir does have a reputation for salts but also can't rule out mold. A clearer picture might help. According to the manufacturer (if it's Ex-Terra) this stuff increases humidity for its animal occupants:
If you just want to go ahead and replace it without testing, I would go with something else formulated especially for plants.
Do you have problems with kettles etc. forming calcium with this water, so that you have to do a vinegar treatment every so often?