If your question/answer has any of the following in it, you will have to enter it a different way:

for <, you must use &lt;

for >, you must use &gt;

for &, I don't remember the key/code

I think the apostrophe might cause this too.

There's some others, but I forget right now (look for symbols in your exercise that might be causing this, delete them and see if the save works, if it does, you'll have to figure out what the code for that symbol is.

It is a little bit annoying, but you will get used to it, or use alternatives, like spell out "less than" or "greater than", or use "+" instead of "&"

I can post a more comprehensive list later, or maybe one of the SM.net guys can post the complete list and also explain why this happens.

Thx for the hint. I realized, i can edit it in text mode, so I'll probably clean up the HTML a little and see what happens .

As there seems to be some heavy development and changes at the moment it's even a good idea to wait a little anyways .

This is what I received from SM:

The problems that you have reported are due to incorrect HTML encoding that you are using.
Please use the entity names as indicated in the chart below and at this site: http://www.w3schools.com/tags/ref_entities.asp

Reserved Characters in HTML
Some characters are reserved in HTML and XHTML. For example, you cannot use the greater than or less than signs within your text because the browser could mistake them for markup.
HTML and XHTML processors must support the five special characters listed in the table below:
Character     Entity Number     Entity Name     Description
"                   &#34;                  &quot;              quotation mark
'                    &#39;                  &apos;             apostrophe (does not work in IE)
&                  &#38;                  &amp;              ampersand
<                  &#60;                  &lt;                    less-than
>                  &#62;                  &gt;                   greater-than
Note: Entity names are case sensitive!

thx for the hint!

It was a different problem: in text-editing mode, I removed some <span>-tag at the end of the field. After that I could edit it in the graphic-mode again .