How are you passing this value from one page to another?
Can you post HTML APEX is generating for this column link?
Can you replicate this issue at apex.oracle.com? If so can you post application link here?
What is your apex version?
Also it's always better to pass ID's (number) through URL and better to fetch actual values on target page using these IDs.
I can try to reproduce it in apex.oracle.com.
Now answers for your questions.
The APEX-Version is 4.2.2.00.11.
On Page 1000 I have a query, that outputs the following:
If I click on the detail-button, the following URL ist used:
The item P1010_KATEGORIE on the page 1010 shows the umlaut not correct:
The column-link on page 1000 is this:
I found a solution for this problem. I create a function that converts the string to his ascii-values. As example:
select STRING_TO_ASCII_F('Hello World!') from dual;
results this: 072101108108111032087111114108100033
This string can used in the URL. On the new page I use another function:
select ASCII_TO_STRING_F('072101108108111032087111114108100033') from dual;
results this: Hello World!
I hope this idea helps other peoples.