Maksim Chmerkovskiy missed Monday night's Dancing with the Stars performance show due to a "personal issue," but according to sources, the pro dancer's issue was with his partner Vanessa Lachey.

At the top of the Dancing with the Stars broadcast, co-host Erin Andrews announced, "Maks is dealing with a personal issue, so Alan is stepping in and dancing with Vanessa tonight," referring to pro Alan Bersten, who got eliminated last week with partner Debbie Gibson.

Fans began speculating whether Maksim was injured or had a family-related problem, but an insider told People that his absence was a direct result of ongoing conflict with Vanessa, a TV personality and former beauty queen.

"There's a big chemistry issue," the insider told the magazine. "They both have big personalities and that's not always the best recipe for a good partnership... They've been fighting a lot."

In fact, last week's performance for "Latin Night" might have been Vanessa's last with Maksim, 37, for Season 25.

"I wouldn't be surprised if Maks doesn't return as her partner for the rest of the season," the insider added.

This news probably comes as a surprise to Dancing with the Stars viewers since the couple appeared to joke around and get along great during the first couple of weeks of rehearsals. Vanessa and Maksim also received consistently high scores for their routines since the start of the competition last month.

A source similarly told Us Weekly, however, that the partnership was "not going well" off-camera.

"There is a lot of fighting going on behind the scenes," the source told Us. "Their personalities are very similar so they disagree about everything. They refuse to give into one another. They're playing nice for the cameras but both are miserable paired with each other."

During Monday night's Dancing with the Stars performance show, a pre-taped package of interactions between Vanessa and Maksim was not even shown. Vanessa's entire segment featured footage of Alan and herself preparing for her guilty-pleasure jazz routine.

Another source told Us that Maksim actually refused to rehearse with Vanessa last week.

In the first two weeks of competition, Vanessa, 36, was shown calling Maksim "the best coach."

The couple also appeared to bond over being parents, as Vanessa is mom to three kids and Maksim just welcomed his first child with Peta Murgatroyd -- who is partnered with Vanessa's husband Nick Lachey -- in January 2017.
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"Nick and Peta are totally fine," the second source assured Us. "Nick is awesome. He's very laid back and sweet. Peta just goes with the flow, she's easy."

This wouldn't be the first time Maksim severely bumped heads with a celebrity partner on Dancing with the Stars.

In October 2015, Maksim told Theo Von and Matthew Cole Weiss for their "Allegedly" podcast of his Season 13 partner, pro soccer player Hope Solo: "[She is] just a sh-tty person."

Maksim made that comment several years after Hope claimed in her memoir that Maksim "manhandled" her during Dancing with the Stars rehearsals as well as slapped her in the face and brutally hit her in the stomach. 

Hope added that Maksim was allegedly "nasty" and "harshly critical" of her -- all of which the pro dancer had denied.