Work your way through the top-right menu, left to right, to find out where the bug might be located. If you see red colored text, it means you already found the difference between Nimbus and the other Ethereum client's tracing result.
If there is no red colored text in the OPCODE section, it means the bug might be located in the TRANSACTIONS section, or in the HEADER section.
Once you locate the bug, you can use the ./build/debug tool to sort things out until there are no more errors and the block passes validation.
Blocks with multiple transactions will have submenus in the navigation bar. Usually, only the first transaction with red colored text is problematic, but it might affect the other transactions. In the OPCODE section, the same thing happens. Perhaps only the first red-colored instruction is problematic, but it will affect the other instructions.
Transactions in the TRANSACTIONS section are marked as: Regular, ContractCreation, or ContractCall. Each kind is processed separately by Nimbus, in different procedures.