Inspector General's Report

One of Trump's go-to arguments about the Mueller Investigation and its predecessor investigation overseen by the FBI and its director, James Comey, was that it was a politically motivated hit job. It was either ordered by President Obama or the so-called Deep State, depending on the day of the week. He and his supporters assert that there was no legal reason to start the investigation and that his campaign had been "spied upon". He characterized the investigations as unconstitutional and treasonous.
According to his circuitous reasoning, the whole investigation was predicated on the "Steele Dossier" (which it wasn't) which turned out to be unreliable (it was) so therefore the whole investigation was illegal. Anyone other than a brainwashed Trump Cultist could see that there were ample reasons to at least investigate. U.S. Intelligence agencies had determined that Russia was interfering with and attempting to influence our election; several members of Trump's campaign, including family members, had an inordinate amount of unexplained contact with Russians; Wikileaks was releasing emails and other documents that had been stolen by Russians; Trump was publicly praising Wikileaks and publicly asking Russia to find Clinton's supposedly missing emails; Trump fired the guy who was overseeing the investigation, Director James Comey and bragged to visiting Russian diplomats that firing Comey took the pressure off him. None of this proved anything, but just what we the public knew was certainly grounds to look into it!

Mueller eventually concluded that the evidence did not support a charge of conspiracy, or that the campaign had actively coordinated with Russia, while acknowledging that the Trump campaign was happy to passively accept the help. They also neglected to notify the FBI of attempts by Russians to coordinate with Trump staffers (including Don Junior). In my view they were saved by laziness.

Once it was clear that Mueller's report contained no "smocking" gun, Trump falsely claimed to have been exonerated (even though the report specifically stated that it did not exonerate him) he continued to attack the investigatory team, threatening to charge them with treason and to "investigate the investigators". Two investigations eventually were started. The first was the investigation that just concluded, by the Justice Department Inspector General authorized by then-Attorney General Jeff Sessions. Internal investigations are what inspector generals of cabinet departments do. Possibly worried that the independent Inspector General might reach conclusions that he and Trump did not like, current Attorney General Barr authorized another investigation. Barr has made statements in line with Trump's paranoid rantings, seemingly concluding that the original investigation was tainted before either investigation was completed.

But enough background.

The Justice Department's Inspector General's report, although identifying a long list of problems with how the investigation was conducted, nonetheless concluded that the Russia investigation was not politically motivated and had an adequate legal basis. Furthermore, no political bias was present in the way it was carried out. Most of the problems that were identified fell within standard FBI procedure; Director Wray is looking at making procedural changes to ensure similar problems do not occur in the future. The biggest issue is the way FISA (Foreign Intelligence Surveillance Act) were obtained. The report indicates that important details were omitted from the warrant applications and in one case an email was altered to omit exculpatory information.

added note:
After rereading the executive summary, the problems identified by the Inspector General can be summarized as follows:

• The application to the FISA court for surveillance of Carter Page was based in part on the "Steele Dossier"
• The application represented Steele, and by extension, his reporting, as reliable, even though Steele himself expressed reservation about at least one of his sources
• The application omitted exculpatory statements by Carter Page
• After Steele provided Mother Jones with information from his report, his status as an official source with the FBI was terminated, but he continued to unofficially provide information to the FBI through Bruce Ohr
• There was a possible perception of conflict of interest in that Bruce Ohr's wife had worked for Fusion GPS, which had hired Michael Steele

Despite allegations that the FISA application neglected to mention that Steele had been hired to do opposition research on Trump by the DNC, the applications contained a footnote to that effect. 

With one exception (an investigator altering an email to delete exculpatory information) the issues identified were mainly procedural, i.e. the investigators were not acting outside of FBI/DOJ procedures, but that the policies and procedures should be tightened up and better oversight of investigations should be included in these changes. 

To reiterate, the Inspector General report debunks the Trumpist conspiracy theory that the Russia investigation was a "Witch Hunt" initiated by Trump's enemies, either the Obama Administration, or some imaginary "Deep State", including twelve, or sixteen, or seventeen "angry Democrats".