Error message when opening form

I am opening a form, previously created in CF6, and get this error message displayed:
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undefined index in ............../libraries/cegcore2/libs/model.php on line 1140

I get this error on each and every form I try to edit.
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Does anyone have a clue what is happening and how I can solve this? I tried reinstalling component, plugin and module but that made no difference.
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Thanks in advance,
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Erik

Hi Erik,
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Which version of CFv6 do you have installed?
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I see this at line 1140 of that file in a function build_flat_list()

 $children = $this->build_flat_list($elements, $element[$this->alias][$this->pkey], ($_depth + 1));

Bob

Hello Bob, it's 6.0.20 .
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Erik

There's an update available, try that

Thanks healyhatman, I installed the update but the error is still there.

Erik

What PHP version are you on?

This one is on 7.0.33

Hi Erik,
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Please post a form backup so that we can take a quick look,.
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Bob

Hello Bob, where/how can I post a fomr backup without it becoming publicly available please?

Hi Erik,
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Please add it to your post in 'Private Content' tags - the padlock icon in the Editor.
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Bob

Hi Erik,
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I deleted your post after downloading as the Private Content tags didn't appear to work.
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I don't have the database table so can't fully test the form but I get no error when loading so it looks as if the error is related to the DB Read.
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Please try adding a Debugger action which should show you the Query that is being generated and the data that lis loaded.
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Bob

Thank you Bob, I'll do. To be continued...

Too bad Bob, that didn't help. In fact, all forms I have in this install show the same error, regardless of wether they read and/or write to tables.

Hi Erik,
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Please use the Contact menu above to ask Max (the owner and developer) and link to this thread.
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Bob

Thank you Bob, I'll do that.

Update: the issue is solved. It turned out there were some missing indexes in some of the Joomla! user database table. Once these indexes were added, the forms could be opened and edited without any error messages again.
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Many thanks for the help Bob and Max !

Erik