Most likely option is that Iran doesn't have full understanding of the technology. The due always to be caught when it's was probably fueling the launcher. The high pressure tolerance or other defects may not have been tested by Iran it is simply didn't understand the technology to make the right design.
Sabotage is extremely difficult as only few people have access to these sites and all of them are highly vetted.
The satellite is most likely unharmed as the fire appears to be in lower level of the launcher and that too, during setting up. But entire rocket has to be dismantled to check for the problem. Considering the number of failures, Iran may never use an used item in any launch.
Iran didn't develop its own Technology but relying on others technology. This will make it very difficult for Iran to understand how things work. If there is any modifications to be done, Iran will not know how to do it.
Safir is underpowered and can't take satellites weighing 400+kg. It can't even take 100kg satellite into stable orbit. This needs that it had to modify it for taking a heavier satellite and putting it in orbit. The modification is difficult if one didn't develop the technology internally.
The failure was in testing the integrity of the assembled rocket or in fueling it. It is most likely related to high pressure related explosion. Considering they rockets use liquid oxygen, there is high chance of explosion of not made appropriate adjustment
How can someone sabotage in a military site in the middle if a desert? Only people who have access to such site is scientists and military. Arresting scientists will result in scientists quitting their jobs and moving out to USA or European countries