On ezplatform 1.6.0rc1 clean install I was unable to publish the content (Blog post content type, title field) if the title contains Ä (https://en.wikipedia.org/wiki/%C3%84) and if the word was never used before (in search index meaning).

reproduce steps:
0. Open webbrowser inspector on network tab if you want to see error message.
1. Go to Content / Content Structure
2. Click on 'create' button, choose `Blog post`
3. fill title field with word containg Ä, i.e. testÄ / Ätest123 (the word should not be used before)
4. fill other mandatory fields: Publication date, Body
5. click on 'Publish'

You will see infinite loading indicator and error in the console/network tab:

Failed to load resource: the server responded with a status of 500 (Internal Server Error) http://domain_.com/api/ezp/v2/content/objects/91/versions/1
Uncaught TypeError: Cannot read property 'ErrorMessage' of null _ezcombo?/bundles/ezplatformui/js/views/dashboard/ez-dashboardblockbaseview.js&/bundles/ezplatformu…:844

the error message is:

<?xml version="1.0" encoding="UTF-8"?>
<ErrorMessage media-type="application/vnd.ez.api.ErrorMessage+xml"><errorCode>500</errorCode><errorMessage>Internal Server Error</errorMessage>
<errorDescription>
An exception occurred while executing 'INSERT INTO ezsearch_object_word_link (`word_id`, `contentobject_id`, `frequency`, `placement`, `next_word_id`, `prev_word_id`, `contentclass_id`, `contentclass_attribute_id`, `published`, `section_id`, `identifier`, `integer_value`) VALUES (:placeholder1, :placeholder2, :placeholder3, :placeholder4, :placeholder5, :placeholder6, :placeholder7, :placeholder8, :placeholder9, :placeholder10, :placeholder11, :placeholder12)' with params [null, 90, 0, 0, &quot;958&quot;, 0, 18, 187, 1477470971, 1, 301, 0]:

SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'word_id' cannot be null</errorDescription>
<Previous media-type="application/vnd.ez.api.ErrorMessage+xml">
<ErrorMessage media-type="application/vnd.ez.api.ErrorMessage+xml">
<errorCode>500</errorCode>
<errorMessage>Internal Server Error</errorMessage>
<errorDescription>SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'word_id' cannot be null</errorDescription>
<Previous media-type="application/vnd.ez.api.ErrorMessage+xml">
<ErrorMessage media-type="application/vnd.ez.api.ErrorMessage+xml">
<errorCode>500</errorCode>
<errorMessage>Internal Server Error</errorMessage>
<errorDescription>SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'word_id' cannot be null</errorDescription>
</ErrorMessage>
</Previous>
</ErrorMessage>
</Previous>
</ErrorMessage>

Note: the second try with the same word containing the Ä character will NOT occur with error, therefore you should use different words to reproduce it many times.

Second part:
If you run the search indexer (php app/console ezplatform:create_sql_search_index) after reproducing steps above, you will see the error:

Indexing Content...
42/47 [=========================>--] 89%

[Symfony\Component\Debug\Exception\ContextErrorException]
Notice: Undefined index: Ai