This happens to me sometimes. Deokotaru told me to delete cache in Blockland folder
I have had this same problem, where my chat gets stuck and I cannot backspace or anything. It seems to be an issue when you accidentally hit the insert button which just so happens to be near the backspace button (at least, thats when it acted up for me)
Deleting the cache.db wouldn't solve the problem, although there are a list of other small reasons this could help with other problems, it doesn't help with this. The only solution I found was mashing the keyboard in rage, or
I get this problem too sometimes. To fix it I usually hit Left Control, tab out of the program, switch back, hit control again and it's fixed.
I hit the windows key, click my desktop, then click back on the blockland window and that seems to work usually