Financially troubled Thurrock Council is considering legal claims against more than 20 other local authorities.
The unitary council in south Essex became effectively bankrupt two years ago after it borrowed and invested hundreds of millions and racked up a £1.5bn debt.
It received valuations for its solar investments from the Association for Public Service Excellence (APSE), which is owned by local councils.
Thurrock "believes that valuations of the solar assets undertaken by APSE, which they allowed the council to rely on, were negligent and overvalued those assets". APSE said it "entirely rejected" Thurrock's statement that valuations "led to significant financial losses to Thurrock with negative impact on taxpayers".