The top navigation is ordered from left to right like the picture below.
Usually you will start from left, then move to the right to find out where the bug might located. If you see red colored text, it means you already found the difference between nimbus and other ethereum client tracing result.
If there is no red colored text in opcode section, it means the bug might be located in transactions section, or in header section.
Once you already located the bug, you can use debug.nim located in /premix to sort things out until there is no more error or it pass block validation.
Block with multiple transactions will have triangle pointing downward in the navigation bar. Usually only the first transaction with red colored text have the problem, but it might affect the rest of other transactions. In opcode section, the same thing happened, perhaps only the first instruction with red colored text have the problem, but it will affect the rest of other instructions.