Multiplus II not staying on charge when I connect my generator

DIY
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1

I have an off grid solar system which has a multiplusII 48/5000/70, recently when I connect my generator, the Multiplus keeps dropping out of the charging process and won’t charge my batteries. I have had no issues with this problem in the past 3 years. Could this be a battery issue? I have 8 x 6V 200ah batteries connected. I have tested the voltage across each battery (when low) all except one are sitting around 5.8V the other is about 4V, could this be my charging issue?

2

What is it doing when you say “dropping out of charge process” it is dropping mains power or just skipping and going straight to float mode?