I tried to reproduce it with that method: Create a wallet in v1.9.5, import some private keys with transactions, then load the wallet to the latest version.
The result is: it loaded without error but the imported private keys aren't included.

Here's the thing,
When I created a new wallet in the latest version, edited the wallet file by adding a funded unrelated address (Puzzle number 79) under "receiving" and "addr_history",
It loaded with the same error as OP and resulted with the same error when tried to broadcast a signed transaction.