very often the backend error appears

Backend errors occur when the server is not working/crashed, does the server return any sort of trace error or is it an abrupt crash? Have you tried removing mods to see if any of them have been causing the issue?

Try taking some mods out of the folder and see what happens, if it fixes the issue then narrow down which exact mod it is

Aight fair enough, just don't leave the thread for too long or the mods will close the thread.

Im getting this constantly, along with laggy item transfers/flee purchases.

Only mods I have are

coremod

AllinOne

lua-fleamarketprices

thatguyxiv-lootgen

Considering that you both have FleaMarketPrices and are experiencing the same issue, try removing flea market prices as it may be breaking the flea and causing a crash whenever you visit the flea market and/or sell

You are probably right, now I see in the server log it happens upon updating flea market prices. I will remove this mod and see if it still happens.

Edit: It is Lua-fleamarketprices. Which is unfortunate, I really like that mod.

Try using Lua's - FleaMarketPriceUpdater if that is the same thing as FleaMarketPrices? I haven't used that mod before but the name is very similar.

Yo dude did you ever get around to removing that mod to see if it fixes it?