The problem with Lotus' 2012 chassis is the front suspension mountings, Germany's Auto Motor und Sport has revealed.
The former Renault team pulled out of this week's Barcelona test when the flaw with its E20 design was discovered, but officials have kept quiet about the details.
Kimi Raikkonen, who was never able to drive the car at the demanding Circuit de Catalunya this week, reportedly suspected there was an issue recently at Jerez, when he reported to engineers an odd feeling relating to the steering.
"It transpires that the problem area is where the front suspension is attached," said the German report.
Auto Motor und Sport said Lotus last year recruited a carbon fibre specialist from Red Bull, who perhaps set a too-ambitious target with the aim of reducing the new car's weight.
Lotus did not confirm the report.
"The good news," said journalist Michael Schmidt, "is that the E20 will be operational to re-start testing next week on time."