I've experienced a similar issue during some plugin development. In short and based on my observations and your brief description, it is most likely your problem is caused by an Action Plugin that is making a modification to the handler "instruction" data before output. At its simplest form it could be a plugin using the Handler Done Event but it could be harder to pin down than this. It could actually be completely unrelated to this and instead be an interface plugin that simply is adding HTML to the output unconventionally. You should test by disabling all plugins and reloading each one and seeing if you can reproduce the bug. It could also be your template/theme, so use the default theme for testing to make sure (it's unlikely but pays to be prudent).
In testing, you should test loading the page cached and not cached. An easy way to do this is to make a small edit to the page which will give you the uncached version. Happy to take a further look at the plugin if you were able to identify which one it was that caused the problem.
The reason I think this is set out in more detail below. The handler produces an 'instruction' list which looks like this:
[28] => Array
(
[0] => tablecell_open
[1] => Array
(
[0] => 1
[1] => left
)
[2] => 99
)
[29] => Array
(
[0] => cdata
[1] => Array
(
[0] => Row 1 Col 3
)
[2] => 100
)
[30] => Array
(
[0] => cdata
[1] => Array
(
[0] =>
)
[2] => 112
)
etc. etc..
The number [2] instruction on each of these arrays is where Dokuwiki stores the character position in each page's underlying .txt file. The 'edit' feature uses this character position when you press 'Edit' in order to decide what text to load into the editor from the source file. If the plugin is adding/modifying the output page, or modifying the instruction list before it displays the page then it will cause these numbers not to correlate to the underlying text that is stored in the .txt file. This results in the part of the text file that is loaded into the editor when you press 'edit' to be the the wrong section. I have observed it providing cut off text, for instance.
Let us know how you go.