Saving very small test pages works fine, but as soon as a larger page is to be saved, nothing is saved and the Wiki says “This topic does not exist”.

When your pages are very large and and contain a lot of complex markup like tables, you could hit a memory limit. Try increasing the memory_limit option in your php.ini.

When it happens even on medium sized pages, you may have a variable size restriction in your PHP install. Known modules to do this are suhosin and varfilter. You need to increase or disable the filter values for the used module in your php.ini. Look for the following values:

varfilter.max_array_depth
varfilter.max_request_variables
varfilter.max_value_length
varfilter.max_varname_length

suhosin.request.max_array_depth
suhosin.request.max_array_index_length
suhosin.request.max_totalname_length
suhosin.request.max_value_length
suhosin.request.max_vars
suhosin.request.max_varname_length