I had the exact error like yours and I also noticed the search products and filter products function on my admin page did not work. I went to Pepipost's support but they were of no help at all. After a week of no response from them, I found the solution here https://www.hostinger.com/tutorials...odify-header-information-headers-already-sent and this fixed those errors.
Pepipost's support later advised me to update to their latest plugin version 3.0.1. When I did that it caused other problems on my back end individual product edit page at the Product Data section which won't display any information now. It's just blank no matter which tab (inventory, shipping, linked products, attributes, variations, etc) I click. Besides that, it also affected the drop down menu on Alidropship Woo plugin > Setting > General > Currency Settings > Additional Currencies. The menu won't response when I click on it, hence can't edit the currencies.
I reported this to Pepipost's support and again, nothing heard from them yet. I may bring this up to ADW team as well.
