5:17 PM PT -- Honchos from the investment company, Dentsu Aegis, tell TMZ ... "As financial partner, we had the customary rights one would expect to protect a large investment. After we exercised our contractual right to take over, and subsequently, cancel the festival, we simply recovered the funds in the festival bank account, funds we originally put in as a financial partner.

"Further, tickets cannot go on sale for an event prior to obtaining a mass gathering permit, which has still not been granted. And we stand by original statement made last week."

Woodstock 50's primary investor is pulling out all the stops to torpedo the 50th anniversary of the landmark rock festival ... so says the man behind the Woodstock brand.

Michael Lang -- who spearheaded Woodstock back in 1969, and is producing Woodstock 50 -- is calling out the investment company, Dentsu, for allegedly trying to sabotage the revival scheduled for later this summer.

According to a letter Lang fired off to Dentsu's Prez and CEO -- obtained by TMZ -- he had concerns from the get-go about such a large corporation being linked to Woodstock ... and it quickly became a thorn in his side.

Lang claims for some unknown reason, Dentsu blocked ticket sales in late April even though the festival had obtained all the necessary permits. He says it also began interfering with the talent lineup ... going so far as to tell bands if they backed out Dentsu would help them land a gig playing the Summer Olympics in 2020 instead.

Lang claims his worst fears were confirmed when Dentsu then surprisingly decided to pull the plug on Woodstock 50 and notified the media it was canceling ... without any legal right to do so.

Even worse -- Lang claims Dentsu "illegally swept approximately $17 million from the festival bank account leaving the festival in peril." Translation ... Lang is suggesting Dentsu pocketed the money.

On the bright side -- Lang says these recent obstacles have led to a "groundswell of support for Woodstock in every conceivable manner, making the prospects of having a successful event a virtual certainty."