TYPO3 Core

Have you set the field size multiplication in the install tool, All Settings? Here's the description:

[multiplyDBfieldSize]
Double: 1-5: Amount used to multiply the DB field size when the install tool is evaluating the database size (eg. "2.5"). This is useful if you want to expand the size of fields for utf-8 etc. For western european sites using utf-8 the need should not be for more than twice the normal single-byte size (2) and for chinese / asian languages 3 should suffice.

This should/might help.

$TYPO3_CONF_VARS["SYS"]["multiplyDBfieldSize"] = '3'
doesn’t changes the behaviour. The header is rejected after the 63-th cyrillic character and a part of the HTML code (!) of the Pagecontent tooll is append instead.

I can confirm that cyrillic letters work properly if you set "multiplyDBfieldSize" to 2 and use utf-8 as BE charset (configurable in istall tool). This is tested with typo3 3.7.0, 3.8.1 and 3.9.0-dev.

Alternatively use windows-1251 and russian language in BE.

This bug relates to misconfoguration of typo3, I think it should be closed.

After changing the "multiplyDBfieldSize" setting, did you do a database COMPARE in the install tool? Just to make sure you didn't miss that (important) step...

Sorry for the newbie posting.
The point was that I expected an “automatic” size correction.
I confirm that the problem was in short field sizes.

This was EBKAC, wrong configuration.