Accrington hang on for 2-1 win.
The U's did get one back in the second half, with Chris Porter netting from the spot, but couldn't get an equaliser despite dominating the late stages.
Accrington were reduced to ten men at the end of the first half, Beckles getting a red card for fouling Brennan Dickenson.
Matty Pearson doubled the lead on the half hour mark, heading home from a right wing corner.
It was the hosts who broke the deadlock, scoring after eighteen minutes through Billy Kee.
The U's have lost 2-1 at Accrington Stanley in their latest away day.
As in Tuesday’s game against Crawley, the U’s started brightly, with Brennan Dickenson twice looking dangerous in the opening minutes.
First, he got onto the end of a Tom Lapslie pass and a Kurtis Guthrie shot was blocked, and then he had a shot off his own blocked following a good ball from Owen Garvan.
The U’s good spell didn’t last long though, and the hosts quickly got a foothold and were getting the ball into the U’s box on a regular basis.
Their first clear cut chance didn’t come until fifteen minutes though, with Jordan Clark having two bites of the cherry from a corner – the first blocked and then a volley pulled wide of the target.
Three minutes later, they got ahead and it was once again from a cross into the penalty area.
Sean McConville put the ball in from the left and finding himself in space twelve yards out, Billy Kee had time to control, look up and blast past Sam Walker to open the scoring.
The U’s went straight up the other end, but Craig Slater’s let foot effort was well over the bar after Harvey Rodgers gifted him a chance.
Home keeper Marek Rodek had to dive full length to preserve his side’s lead, when Owen Garvan’s wide free kick evaded everyone and looked to be heading towards the corner.
There was an early substitution for U’s boss McGreal on twenty five minutes, with Chris Porter replacing Denny Johnstone, who suffered a knee injury.
On the half hour, it went from bad to worse for the U’s when Matty Pearson doubled the Accrington lead.
Shay McCarton’s free kick was saved by Walker but couldn’t keep out Pearson’s far post header from the resulting corner.
Stanley sensed blood at this stage, and Clark was next to get into a decent position, shooting wide after cutting inside Kane Vincent-Young.
There was drama a minute before half time, when the hosts were reduced to ten men.
Dickenson beat Omar Beckles to the ball, the home defender caught him and the referee harshly issued a red card, rather than the expected yellow one.
McGreal made his intentions clear at half time, bringing on Matt Briggs and Drey Wright to bolster the attacking options.
But it was the hosts, despite being down to ten men, that started the second period the brightest.
Elokobi was booked giving a foul away to prevent an Accrington attack a minute in and then Walker made a comfortable save from Kee’s header from a corner.
The game should have been dead and buried on the hour, with the hosts missing a glorious chance after a great advantage from the referee, Mr Clark.
Alex Wynter committed the foul, but the loose ball fell to McCarton who rolled his effort past the post when through on goal.
Kee and Brown were next to try their luck but their shots were blocked and put wide respectively.
The U’s got an unlikely lifeline with fifteen minutes remaining, when Garvan's superb volleyed pass found the run of sub Wright.
Wright reached it first but before he could think about getting a shot away, he was fouled and the referee pointed to the spot.
Fellow sub Porter stepped up and firmly sidefooted into the bottom corner to score his tenth goal of the season.
The final quarter of an hour saw the U’s camped in the home half and Porter almost got a leveller but diverted Elokobi’s overhead kick narrowly over the bar.
He turned provider next, flicking on for Guthrie, but the shot was easy for the home keeper and his side remained in front.
Porter was in the thick of things as he tried to get another goal, but a near post attempt rolled just wide before Guthrie repeated things with a near post header.
Slater then cut inside but couldn’t keep his shot inside the post, and that was the final chance for the visitors.
The pressure on the hosts was immense in that final fifteen minutes, but they stayed strong and the U’s were left with nothing to show for their late efforts.
It would have been harsh on Accrington had they conceded an equaliser and the U’s will have to look to Tuesday night’s game against Wycombe to put things right.